calibre MOBI Email Delivery Stopped: What to Use Instead

If calibre previously emailed MOBI books to your Kindle but Amazon now rejects them or nothing arrives, the most likely cause is not a broken MOBI conversion. Amazon changed its Send to Kindle intake and no longer accepts MOBI attachments through Kindle email delivery. In most cases, the practical replacement is EPUB. Other possible causes include an outdated format list in calibre, an unapproved sender address, a confirmation request from Amazon, a damaged EPUB, an email authentication failure, or security software blocking calibre. Work through the checks below in order, and stop as soon as one small test EPUB appears in your Kindle library.

1. Confirm the Symptom With a Small Safe Test

Begin by separating an Amazon format rejection from a general calibre, email, or account problem. Do not test with a large illustrated book, a downloaded newspaper, or a heavily formatted document. Those files introduce extra variables and can take longer to process.

1.1 Check Whether MOBI Is the Attachment Being Sent

Select the affected book in calibre and look at the Formats field in the Book details panel. If the book contains MOBI and EPUB, do not assume calibre will automatically choose EPUB. The format selected for email delivery depends on the configured format list for that recipient.

Right-click the Connect/share button and examine the email delivery options. If calibre sends a MOBI attachment and Amazon rejects it, that is the expected result under Amazon's current Send to Kindle rules. It does not prove that calibre conversion is broken or that the local MOBI file is unreadable.

1.2 Send One Small EPUB

Choose a small, ordinary EPUB that opens correctly in calibre's E-book viewer. Ideally, use a text-focused book without complex tables, embedded media, unusual fonts, or a fixed layout. Send only that EPUB to your Kindle address.

Success means the document appears in your Kindle library or on the intended device after Amazon processes it. If that happens, stop changing settings. You have confirmed that calibre, your outgoing email account, Amazon authorization, and EPUB delivery are working. The original problem was the MOBI attachment or the original book file, not calibre as a whole.

If the test EPUB also fails, check your inbox, spam folder, and the job details in calibre before trying another conversion. Amazon may have sent a rejection explanation or a message requiring confirmation.

2. Replace MOBI With EPUB in calibre Email Delivery

The main calibre MOBI email delivery stopped fix is to change the allowed formats for the Kindle recipient. This setting is separate from the formats stored in your library and separate from the format you use for USB transfers.

2.1 Change the Recipient's Format List

1. Open calibre.
2. Go to Preferences.
3. Open Sharing books by email.
4. Select the row containing your Send to Kindle email address.
5. Remove MOBI from that recipient's format list.
6. Add EPUB and place it first if the interface allows format ordering.
7. Apply the changes and restart calibre if prompted.

Send the small EPUB again. Success means calibre attaches an EPUB rather than silently choosing a MOBI copy from the same library record. Once the document arrives, leave the recipient configuration alone and test the original title as EPUB.

2.2 Set EPUB as the Preferred Output Format

Open Preferences > Interface > Behavior and review Preferred output format. Select EPUB if Kindle email delivery and EPUB-based workflows are now your main use case.

This preference influences several calibre actions, including some save, conversion, and news-download workflows. It does not necessarily override every recipient-specific email rule, so you should still change the Kindle recipient's format list directly.

Success means new conversions or generated news editions are produced as EPUB when the workflow uses the preferred output setting. Stop adjusting conversion defaults once calibre creates the intended EPUB and the small test succeeds.

2.3 Understand Old MOBI and Newer Kindle Formats

MOBI is an older Kindle-related format, but the terminology can be confusing. Some MOBI files contain older MOBI7 content, while other files may contain newer KF8 content or a combination of old and new representations. AZW3 is also associated with newer Kindle formatting. These distinctions can matter for local reading and USB transfer, but they do not restore MOBI email delivery.

Amazon's Send to Kindle service accepts EPUB as an input and converts it into a suitable Kindle document for supported devices and apps. You do not need to email AZW3, a newer-style MOBI, or another Kindle-specific output. In fact, trying a succession of Kindle formats can hide the simple issue: the email service expects a supported source format such as EPUB.

3. Check Amazon and Email Settings Directly Related to Delivery

If calibre sends EPUB but the book never arrives, inspect the Amazon account and outgoing email path. A successful calibre job only proves that calibre handed the message to the configured mail server. It does not prove that Amazon accepted the attachment.

3.1 Approve the Exact Sender Address

In Amazon's Manage Your Content and Devices area, open the personal document settings and review the Approved Personal Document Email List. The exact address used by calibre must appear there.

Check the address carefully if you recently changed email providers, created an app-specific password, switched from one SMTP account to another, or edited the From address. An alias may deliver normal email successfully while still failing Amazon's sender authorization check.

Success means a new EPUB from that exact sender reaches the Kindle library. Once it does, do not keep adding alternate addresses or changing the Kindle destination.

3.2 Verify the Kindle Destination Address

Confirm that calibre is sending to the current Send to Kindle address associated with your account or device. A renamed Kindle address, old device entry, typographical error, or address belonging to another Amazon account can make the problem look like a conversion failure.

If you own multiple Kindles, send the test to the destination you actively use. Then check the account's library, not only the home screen of one reader. A document may enter the cloud library before it downloads to a particular device.

3.3 Read Amazon's Confirmation and Rejection Messages

Search the inbox of the sending account for messages from Amazon or Kindle support. Also check spam, junk, promotions, and filtered folders. Depending on the account and sending pattern, Amazon may ask you to confirm a document delivery. If you ignore that message, the attachment will not appear even though calibre reports that the email was sent.

A rejection message can also identify an unsupported attachment or conversion failure. Treat that message as more useful than repeatedly pressing Send. If Amazon identifies MOBI as unsupported, switch to EPUB. If Amazon rejects a particular EPUB while the small test works, troubleshoot the book file rather than the account.

3.4 Test the Outgoing Email Account

In Preferences > Sharing books by email, use calibre's email test function if available. Verify the SMTP server, port, encryption setting, username, password, and From address. Some providers require an app password or another provider-specific authentication method.

Success means calibre completes its email test and a normal message can leave the account. If the SMTP test fails, Amazon has not received anything, so changing EPUB conversion options will not help. Fix the email login or provider requirements first.

4. Check the Book, Metadata, and Conversion Path

If one test EPUB arrives but a particular title is rejected, the delivery system is working. Focus on the affected library record and source file.

4.1 Confirm That the EPUB Opens Before Sending

Right-click the View button and explicitly open the EPUB format. Move through several chapters, use the table of contents, and confirm that text and images load. Opening the MOBI version does not validate the EPUB attachment.

If the EPUB does not open, remove only that broken format from the book record and create a fresh EPUB from a lawful, readable source file. Do not delete the entire book or library. Success means the newly created EPUB opens locally and then passes the same email test.

4.2 Avoid Unnecessary MOBI-to-EPUB Conversion

When a clean EPUB source already exists, send it directly. Repeated conversion can simplify styling, alter navigation, reduce image quality, or preserve defects from the intermediate format. If the only available source is MOBI, calibre can convert an unprotected file to EPUB, but inspect the result before emailing it.

This article does not recommend bypassing DRM or other access controls. calibre cannot convert protected content through its standard conversion workflow, and retailer or publisher restrictions should be respected.

4.3 Review Metadata and File Names

Ordinary metadata errors rarely explain a complete service-wide stop, but unusual data can cause trouble with an individual document. Check the title, author, language, cover, and identifiers. Remove malformed HTML from comments if it was copied from an unreliable source, and avoid extremely long titles or file-name templates.

If metadata was downloaded by a plugin or imported from an old OPF file immediately before the problem began, compare the affected record with the working test book. Success means a minimally edited copy converts and sends correctly. Stop there rather than rewriting all library metadata.

4.4 Check News and Automated Workflows

If scheduled news delivery stopped, confirm that the downloaded edition is being generated as EPUB rather than MOBI. Set EPUB as the preferred output format, download one edition manually, open it in the viewer, and send it manually before re-enabling automatic delivery.

The calibre Content server can also provide another route for accessing library books or downloaded news through a browser. However, changing Content server ports, accounts, or network exposure will not fix Amazon's rejection of a MOBI email attachment. Treat it as an alternative delivery method, not a MOBI email repair.

4.5 Temporarily Disable Format-Changing Plugins

A plugin can affect metadata, conversion, or device behavior, but plugins are not the first suspect when Amazon rejects MOBI. If the small EPUB works and only plugin-processed books fail, repeat the test with the relevant plugin disabled or start calibre without third-party plugins.

Success means an unmodified EPUB sends correctly. Re-enable plugins one at a time until the failure returns. Do not remove unrelated plugins or rebuild the library without evidence that they participate in the email job.

5. Check Operating System, Security, Network, and USB Factors

Operating system issues matter when calibre cannot send any email or when you choose USB as an alternative. They do not change Amazon's accepted email formats.

5.1 Firewall and Antivirus Checks

Security software can block calibre from opening an SMTP connection. This is most likely when the job fails before the message leaves your computer, calibre reports a connection or timeout error, or email delivery stopped after a security-suite update.

Allow calibre to make outbound connections through the firewall or antivirus product. Avoid disabling protection permanently. On managed computers, ask the administrator whether SMTP connections are blocked.

Success means calibre's email test completes and the job log shows that the mail server accepted the message. If Amazon then rejects MOBI, return to EPUB rather than changing more firewall rules.

5.2 Permissions and Cloud-Synced Libraries

A read-only source file, restricted temporary directory, or interrupted cloud-sync operation can prevent calibre from creating the attachment it intends to send. Look for permission-denied, file-not-found, or locked-file messages in the job details.

For a controlled test, copy one unprotected source book to a normal local folder, add it to a temporary calibre library, convert it to EPUB, and send it. On Windows, avoid protected system folders. On macOS, verify that calibre can access the folder. On Linux, check ownership and write permissions for the user running calibre.

Success means the local test completes without file access errors. If it works, investigate the original folder or sync service rather than reinstalling calibre.

5.3 Network Reliability

A network failure usually produces an SMTP connection, DNS, certificate, or timeout error. It does not normally produce an Amazon message specifically rejecting MOBI. Test from a stable connection and temporarily disconnect a VPN or restrictive proxy only if the logs show a connection problem.

Once calibre reports that the SMTP server accepted the test message, stop changing network settings. Continue with the Amazon approval and attachment checks.

5.4 Use USB Transfer as an Alternative

USB transfer avoids the Send to Kindle email intake and can be useful when you need a document immediately or when an EPUB repeatedly fails Amazon's conversion. Connect the Kindle with a data-capable cable, wait for calibre to detect it, and use Send to device or Send specific format to.

Device support and suitable local formats vary by Kindle model and transfer method. If calibre offers automatic conversion for the detected device, review the proposed output rather than assuming that the same EPUB used for email should simply be copied unchanged.

On some recent Kindle and Kindle Scribe models, particularly with macOS, USB file access may use Amazon's current USB management workflow rather than appearing like a traditional removable drive. Follow the procedure for your model and operating system.

Success means the book appears under the device's documents or library view and opens normally. USB success does not mean MOBI email delivery has been restored. It only confirms that the book can be transferred through a different path.

6. Use calibre Logs When the Basic Fix Does Not Work

Logs help determine whether the failure happened during conversion, attachment creation, SMTP submission, Amazon processing, or USB detection.

6.1 Read the Email Job Details

After sending a book, click the Jobs indicator in calibre and open the details for the email task. Look for the attachment format, destination address, mail-server response, authentication errors, timeouts, and final job status.

• If the attachment is MOBI, correct the recipient format list.
• If authentication fails, fix the outgoing email account.
• If the server accepts the message, check Amazon's response and sender approval.
• If only one EPUB fails, inspect or recreate that EPUB.

Save the complete log before changing anything. The final line alone may omit the cause.

6.2 Inspect Conversion Job Details

If calibre created an EPUB immediately before sending it, open the conversion job details. Warnings do not always make a book unusable, but missing source files, parsing failures, write errors, or an aborted conversion require attention.

For difficult conversion problems, use calibre's conversion debugging options to preserve intermediate output. This is useful only when the EPUB itself is malformed or displays incorrectly. It is not necessary when a known-good EPUB works and only MOBI is rejected.

6.3 Use Debug Mode or calibre-debug Selectively

Advanced users can restart calibre in debug mode from the Preferences menu and reproduce the failure once. The command-line tool calibre-debug can also start the graphical interface with diagnostic output. Device-detection debugging is useful when a USB-connected Kindle does not appear, but it will not diagnose an Amazon email-format rejection.

Use calibre-debug --debug-device-driver for device detection problems and calibre-debug -g when broader GUI diagnostics are requested. On macOS, calibre command-line tools are stored inside the application bundle unless they have been exposed separately. Stop once the log identifies a specific component. Do not run unrelated commands or edit calibre's internal files.

7. Run a Clean Temporary Test Before Reinstalling

Reinstalling calibre rarely fixes an Amazon policy change or an unapproved sender. A clean test is safer and provides better evidence.

1. Create a temporary calibre library in a normal local folder.
2. Add one small, unprotected EPUB that opens correctly.
3. Configure one Kindle email recipient with EPUB as its allowed format.
4. Configure or test the outgoing email account.
5. Send the EPUB once.
6. Check calibre's job details and the email inbox for Amazon messages.
7. Wait for Amazon processing before repeating the test.

If the temporary test works, the calibre installation is functional. Compare the temporary recipient settings, source file, plugins, and library location with the original setup. Do not delete the original library.

If the clean test fails before SMTP submission, focus on the email account, firewall, permissions, or network. If SMTP submission succeeds but Amazon rejects the attachment, focus on the supported format, approved sender, confirmation message, and EPUB quality.

8. Quick Fix Checklist

• Stop emailing MOBI files to your Send to Kindle address.
• Set the Kindle recipient's calibre format list to EPUB.
• Set EPUB as calibre's preferred output when appropriate.
• Open the exact EPUB in calibre before sending it.
• Test one small, simple EPUB instead of a large book.
• Verify the exact sending address is approved by Amazon.
• Confirm the destination is your current Send to Kindle address.
• Check inbox and spam folders for Amazon confirmation or rejection messages.
• Open calibre's email job details and identify where the process stopped.
• Fix SMTP authentication before changing conversion settings.
• Temporarily test without a relevant conversion or metadata plugin.
• Use a local temporary library to rule out permissions or cloud-sync locks.
• Use USB transfer when email delivery is unavailable or unsuitable.
• Stop changing settings as soon as a known-good EPUB arrives and opens.

9. Frequently Asked Questions

9.1 Why Did calibre Suddenly Stop Emailing MOBI Files to Kindle?

Amazon ended MOBI acceptance for Send to Kindle delivery. calibre may still create, store, open, or transfer MOBI files locally, but that does not make MOBI a supported email attachment. Change the configured format for the Kindle email recipient to EPUB.

9.2 Should I Convert MOBI to AZW3 Before Emailing It?

No. AZW3 is useful in some local Kindle and USB workflows, but it is not the appropriate replacement attachment for Send to Kindle email. Use EPUB for email delivery when the document and Amazon service support it.

9.3 Why Does calibre Say the Email Was Sent When the Book Never Appears?

A successful calibre email job can mean only that the outgoing mail server accepted the message. Amazon can still request confirmation, reject the attachment, reject the sender, or fail to convert the file. Check Amazon's messages and the Approved Personal Document Email List.

9.4 What If Amazon Rejects an EPUB That Opens in calibre?

First send a different small EPUB. If the second file works, the original EPUB likely contains markup, navigation, image, font, or packaging that Amazon's converter does not accept. Create a fresh EPUB from the lawful source, simplify unnecessary conversion options, and test the result locally before sending it.

9.5 Does Setting EPUB as the Preferred Output Fix Everything?

No. It helps conversions, saved output, and some generated-content workflows, but the Kindle recipient's email format list must also allow EPUB. Sender approval, SMTP authentication, Amazon confirmation, and source-file quality remain separate checks.

9.6 Can I Still Put Books on Kindle by USB?

Yes, USB remains an alternative for compatible devices and file types. Use calibre's detected-device actions when available, or the transfer workflow supported by your Kindle model and operating system. Remember that USB transfer bypasses Amazon's email processing, so cloud storage, synchronization, formatting, and personal-document features may differ.