Unfortunately, this is not unheard of and there's a number of reasons this can happen, such as:

• Your Mailbox(es) are bloated with years and years worth of email data
• an unresponsive macOS at the time (usually due to issues with Mail Data, but also possibly for unrelated reasons)
• a somewhat corrupted inbox
• macOS being unable to find (or having delays in finding) the relevant message data
• unintended consequences due to blocked users or Mail rules

Rebuilding mailboxes was certainly a good place to start. Since that didn't resolve your issue, I would try manually reindexing your mail data.

To do this, follow these steps:

1. Quit Mail
2. Go to Finder
3. Press shiftcommandG
4. After the Go to Folder window appears, type (or copy and paste) the following file path: ~/Library/Mail/V9/MailData

NOTE: For users not running macOS Monterey, the "V9" part of the above file path will be different - e.g. for macOS Ventura it will be V10, for macOS Big Sur it will be V8.

5. Press return
6. The MailData window appears, containing a number of files and folders relating to the Mail app
7. Make sure you sort the view by Name (if it isn't already)
8. Make a backup of the Envelope Index file and the ExternalUpdates.storedata file (copying them to your Desktop is fine) and then trash them
9. You'll probably see other files starting with "Envelope Index" and "ExternalUpdates" - if so, also trash these files
10. Open Mail again
11. When prompted to re-import your messages, click OK. (Note: You may get a warning that the index is damaged and that Mail has to quit.)
12. Be patient while Mail imports your messages. This can take anywhere from a few minutes to a number of hours, depending on how much data you actually have stored locally on your Mac.

Once complete, test Mail to see if this has resolved your issue.

NOTE: If you have any unexpected problems, repeat Steps 1 to 6 above and then copy back the files you backed up at Step 8.

Unfortunately, this is not unheard of and there's a number of reasons this can happen, such as:

• Your Mailbox(es) are bloated with years and years worth of email data
• an unresponsive macOS at the time (usually due to issues with Mail Data, but also possibly for unrelated reasons)
• a somewhat corrupted inbox
• macOS being unable to find (or having delays in finding) the relevant message data
• unintended consequences due to blocked users or Mail rules

Rebuilding mailboxes was certainly a good place to start. Since that didn't resolve your issue, I would try manually reindexing your mail data.

To do this, follow these steps:

1. Quit Mail
2. Go to Finder
3. Press shiftcommandG
4. After the Go to Folder window appears, type (or copy and paste) the following file path: ~/Library/Mail/V9/MailData

NOTE: For users not running macOS Monterey, the "V9" part of the above file path will be different - e.g. for macOS Ventura it will be V10, for macOS Big Sur it will be V8.

5. Press return
6. The MailData window appears, containing a number of files and folders relating to the Mail app
7. Make sure you sort the view by Name (if it isn't already)
8. Make a backup of the Envelope Index file and the ExternalUpdates.storedata file (copying them to your Desktop is fine)
9. You'll probably see other files starting with "Envelope Index" and "ExternalUpdates" - if so, trash these files
10. Open Mail again
11. When prompted to re-import your messages, click OK. (Note: You may get a warning that the index is damaged and that Mail has to quit.)
12. Be patient while Mail imports your messages. This can take anywhere from a few minutes to a number of hours, depending on how much data you actually have stored locally on your Mac.

Once complete, test Mail to see if this has resolved your issue.

NOTE: If you have any unexpected problems, repeat Steps 1 to 6 above and then copy back the files you backed up at Step 8.

Unfortunately, this is not unheard of and there's a number of reasons this can happen, such as:

• Your Mailbox(es) are bloated with years and years worth of email data
• an unresponsive macOS at the time (usually due to issues with Mail Data, but also possibly for unrelated reasons)
• a somewhat corrupted inbox
• macOS being unable to find (or having delays in finding) the relevant message data
• unintended consequences due to blocked users or Mail rules

Rebuilding mailboxes was certainly a good place to start. Since that didn't resolve your issue, I would try manually reindexing your mail data.

To do this, follow these steps:

1. Quit Mail
2. Go to Finder
3. Press shiftcommandG
4. After the Go to Folder window appears, type (or copy and paste) the following file path: ~/Library/Mail/V9/MailData

NOTE: For users not running macOS Monterey, the "V9" part of the above file path will be different - e.g. for macOS Ventura it will be V10, for macOS Big Sur it will be V8.

5. Press return
6. The MailData window appears, containing a number of files and folders relating to the Mail app
7. Make sure you sort the view by Name (if it isn't already)
8. Make a backup of the Envelope Index file and the ExternalUpdates.storedata file (copying them to your Desktop is fine)
9. You'll probably see other files starting with "Envelope Index" and "ExternalUpdates" - if so, trash these files
10. Open Mail again
11. When prompted to re-import your messages, click OK. (Note: You may get a warning that the index is damaged and that Mail has to quit.)
12. Be patient while Mail imports your messages. This can take anywhere from a few minutes to a number of hours, depending on how much data you actually have stored locally on your Mac.

Once complete, test Mail to see if this has resolved your issue.

NOTE: If you have any unexpected problems, repeat Steps 1 to 6 above and then copy back the files you backed up at Step 8.

Unfortunately, this is not unheard of and there's a number of reasons this can happen, such as:

• Your Mailbox(es) are bloated with years and years worth of email data
• an unresponsive macOS at the time (usually due to issues with Mail Data, but also possibly for unrelated reasons)
• a somewhat corrupted inbox
• macOS being unable to find (or having delays in finding) the relevant message data
• unintended consequences due to blocked users or Mail rules

Rebuilding mailboxes was certainly a good place to start. Since that didn't resolve your issue, I would try manually reindexing your mail data.

To do this, follow these steps:

1. Quit Mail
2. Go to Finder
3. Press shiftcommandG
4. After the Go to Folder window appears, type (or copy and paste) the following file path: ~/Library/Mail/V9/MailData

NOTE: For users not running macOS Monterey, the "V9" part of the above file path will be different - e.g. for macOS Ventura it will be V10, for macOS Big Sur it will be V8.

5. Press return
6. The MailData window appears, containing a number of files and folders relating to the Mail app
7. Make sure you sort the view by Name (if it isn't already)
8. Make a backup of the Envelope Index file and the ExternalUpdates.storedata file (copying them to your Desktop is fine) and then trash them
9. You'll probably see other files starting with "Envelope Index" and "ExternalUpdates" - if so, also trash these files
10. Open Mail again
11. When prompted to re-import your messages, click OK. (Note: You may get a warning that the index is damaged and that Mail has to quit.)
12. Be patient while Mail imports your messages. This can take anywhere from a few minutes to a number of hours, depending on how much data you actually have stored locally on your Mac.

Once complete, test Mail to see if this has resolved your issue.

NOTE: If you have any unexpected problems, repeat Steps 1 to 6 above and then copy back the files you backed up at Step 8.