A short answer is, did you have a look at this article and make sure you checked everything? This may be a Retention / Sychonization & Storage issue.

Did you ever monitor the Activity Manager?

If you never changed your profile, I cannot explain why different versions behave differently with that profile. On the other hand, it seems you are using development versions only (or at least reporting the issue for), so some oddities can be expected.

Thanks very much for the suggestions. I have followed up by going through most if not all of the alternatives mentioned in the "Disappearing Mail" article, and I made sure that all of my Macs are running the stable version of TB (52.3). Furthermore, the items have not been quarantined by my anti-virus software.

Using the Outlook Web App, I was able to access the missing messages. I forwarded the 2 messages that are disappearing on TB to myself, and that allowed me to receive and copy the forwarded messages to a private TB mailbox.

Nevertheless, the problem persists. If I recover the original deleted items to my Outlook Web App Inbox, they appear there, allowing Reply, Forward, etc. However, the items do not concurrently appear in the TB Inbox. Accessing the messages from TB actually deletes the messages from the OWA Inbox, and we are back at Square One.

- Did you make sure all retention settings are correct, both general and per folder, and for all Thunderbird clients? - Did you make sure the client at the MacAir has the proper (and preferably matching) settings in Account Settings > Server Settings > When I delete a message ?

It seems like both messages were marked for deletion at that client, perhaps for a brief moment after changing one of the above settings and back, or when using a developer build, or for another unknown reason. I do assume both TB clients are set up as IMAP and this is not about the last 2 messages arrived, correct?

If you are sure the above settings are correct and identical to what the other client uses, you could also remove (or move) the Inbox.msf file at the MacAir while Thunderbird is closed, and then restart Thunderbird. I’m not sure if the message was missing there, but think this action will bring it back on both clients - the .msf file will rebuild itself.

The file should reside in the Profiles/ImapMail/[imap_server_name] folder. You can find the profile folder using Help > Troubleshooting information > Show in Finder.

If that does not help, try deleting/moving both the Inbox.msf and Inbox files. When launching Thunderbird, all messages should be synced with the server as if the account was newly set up (so this can take a few minutes).

Also see this article for more info.

I appreciate your perseverance, but I still have the problem. I've followed the leads in the articles that you referenced, and I believe that all the settings are correct. I ditched the Inbox and Inbox.msf for the exchangeimap server, as well as my local versions (which contain nothing), but the same symptoms reappear.

About the only thing that the 2 disappearing messages have in common is that both signature blocks contain an image (i.e., a logo). I asked one of the senders to send a test message without the logo but have yet to get a response. I haven't looked at the full headers; might they be useful?

An observation: Deleted Items is not a regular Thunderbird folder. I believe it is a standard Outlook/Exchange folder. As such, any messages being moved into it might be being moved by Exchange rather than by Thunderbird.

I generally find that if a server offers its own folders such as Sent Items, Deleted Items etc, life is much easier if you adjust Thunderbird to use them rather than trying to coerce the server into using Thunderbird's default folder names.

Of course, if you have already set Thunderbird to use Sent Items, Deleted Items etc then it is a bit harder to work out who or what is doing the unwanted moves.

Well spotted. Have you ever renamed the folder on the MacAir, or perhaps hidden it by unsubscribing in Thunderbird?

There is also this bug that may cause messages to get lost. If you suffer it, you may find many .msf files in the Profiles/ImapMail/[imap_server_name] or subfolder. The solution would be to delete the panacea.dat (a cache file) in the same folder when Thunderbird is closed. That may help resolve various issues.

If that does not fix it, you could add a second profile and set up the account there, just to see the issue is profile related or not.

With regard to missing images, I would think of security software blocking images either by their web link or local content, but not to mark entire messages as deleted, unless it does a bad job. What security software do you use, and does the issue occur in Windows Safe Mode with networking enabled?

Looking at and comparing full headers if possible could be useful, when needed.

Thanks, Tonnes and Zenos. I've tried all your suggestions, and the problem persists. Deleting panacea.dat may have helped, but the messages still wind up in the Trash.

I have localized the problem as having to do with images, both in signatures and as attachments. I received another email with an image in the signature this morning, and it went straight to the Trash. After deleting panacea.dat, I recovered the item using Outlook Web App; this time the item was not deleted. I should add that this item was received by another Mac using TB52 on another server, and that computer did not have the problem.

This morning, I sent an email from my iPhone containing 3 photos. My cc'ed copy went straight to the Trash on my MacAir with TB52 (not to Deleted Items). I was able to recover that item from the Trash to my Inbox without using Outlook Web App. That is better than before, but the problem is still not solved.

P.S. I asked the sender of one of the original problem emails to re-send without the signature block, and it was received without any problems.

Thunderbird should not delete messages based on signatures or their presence of images. The only possibility would be that it uses its adaptive Junk log in some aggressive way, so either disable Junk filtering in Account Settings > Junk Settings, and enable its logging in the Global Junk settings from there using the button at the bottom or in Preferences > Security panel > Junk tab, OR antivirus software is scanning the messages because it’s allowed in the same Security panel > Antivirus tab - disable Allow antivirus clients to quarantine individual incoming messages.

If that doesn’t do it, this may lead to another thread where security or antivirus software is left out of the discussion and may be the culprit, which usually happens for Windows. Could there be some ZoneAlarm or Kaspersky product on the MacAir (if available for Mac), and can you completely remove it as a test, not just disable it?

Thanks for the many suggestions. I've been with TB since version 1.0 and Netscape before that, never having a problem that dug this deep into a support forum. This problem (still not fixed) and the material at https://en.wikipedia.org/wiki/Mozilla_Thunderbird has convinced me to move on to Apple Mail. I've migrated my local files folder-by-folder, as neither Apple Mail Import or Thunderbird Export routines were up to the task. Interestingly, I picked up a whopping 25GB of disk space in the process. Some of that was due to deleting messages with large attachments; my guess is that most of it was due to storage inefficiencies that have occurred after 20+ years with the same email platform. Thanks again for your support.

Moving on to Apple Mail is no solution, but would be a workaround. Also you don’t need to switch, but you can of course use multiple clients instead.

I have not seen a reply to my last question, so I would suggest to check any antivirus or "shield" software once again.

Also and as said before, if Thunderbird deleted messages, it would be either because of its own Junk filter, or its regular message filters. Both contain logging options, so you should see entries for them when enabled. There is no point in "guessing". ;)

Fwiw: I have been using the products as long as you and never seen such issues, most likely because I don’t use antivirus or other so-called security software that is able able to mess up incoming messages.

Finally, it would be great if you could prove Thunderbird is at fault in order to have a bug fixed and help out other users. It would be a waste to see that opportunity go.

Again, I really appreciate your feedback. If I had the time to continue to try to fix the problem, I would have done so and not switched to Apple Mail; however, that isn't the case. I rely on email, as most people do, and I need a reliable client. While I have migrated my files via brute force over a period of several days, I haven't disabled TB, although at this point, it is a backup.

My guess is that this was not a systemic TB52 problem but corrupt files in my copies and/or corrupted messages that may have induced a hidden bug. In hindsight, I probably should have done a clean re-install rather than chase down incremental fixes.

The difficulty that I had with migration suggests that some local-mailbox messages were damaged; for example, dates were missing on some very old files. One mailbox containing a few hundred messages from ~2000 to 2017 hung up after loading items up until 2009; I created a new copy, deleted everything before 2010, and migrated it without problems. Then I deleted everything in the original mailbox after the point of hangup, migrated that without trouble, and concatenated it to the new version. I lost a few dozen messages at most, including the corrupt one, and the damage was minimal.

My local items have gone from TB to Outlook/Exchange and from there to Apple Mail. Buggy files have either been cleaned up or discarded, and, as noted earlier, they take up less memory than before. Apple Mail appears to be a satisfactory solution.

I would be quite interested in seeing the troubleshooting information. There has been much discussion here about how things should work, but there is really no information as to the actual settings being used.

Please add the troubleshooting information to your post To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.