re :I have my Thunderbird settings so that any emails I actually delete don't go into the trash but simply disappear.

That usually means they are by-passing the server Trash folder.

re:The interesting thing about these emails is that, when they go into the trash folder, they stay there. If that 'Trash folder is the server default Trash folder, then do not leave those emails in the Trash because AOL have an auto empty policy.

• https://help.aol.co.uk/articles/spam-and-trash-folders-are-regularly-deleted-in-aol-mail

Why are they getting put in the TRash.

This can occur if you have set up a Message Filter which has an action to move emails to Trash. But it's conditions are not as desired because they are effecting most emails.

This can also occur if you view your emails via another computer, laptop or via a phone. If you have a set up to see emails on eg: phone and it's using a Pop protocol, then it's possible the phone is downloading mail and auto deleting mail off server upon download. Hence server is putting emails in Trash and then you see them in TRash via the Imap account in Thunderbird. Check those other mail accounts to ensure if using Pop then select to 'Leave message on server' so it's still in the Inbox. Or maybe you are using Imap to view emails but did not realise that deleting emails off phone would effect the server and therefore effect other Imap connections.

AOL is also now using Yahoo servers. Have you got server settings using aol or aol on yahoo ?

Thanks for coming back to me, Toad-Hall. I don't use Pop3 protocols on any of my email accounts - they are all IMAP. But whilst my main account and one other have this annoying habit of email migrating from the inbox to the trash, the others do not.

I do access my emails from one of three Macs (where I always use Thunderbird), or on my iPhone or iPad, where I use Mac Mail. Regards Roger