Rob, the first thing we need to understand is how your Mail client is connecting to Fastmail. Do you have it set up to use IMAP or POP?
If you are using POP (not recommended), the problem is usually a corrupted client. If you are using IMAP (which is probably the case), please start by reading this from Fastmail help:
Problems deleting email
So there are two ways that IMAP email clients may delete messages:
- The old IMAP technique: The message is marked for deletion by the email client (which adds a flag to the message and then pushes that change to the Fastmail IMAP server), but is not actually removed from your Inbox until a purge messages marked for deletion action is triggered.
- The newer technique: The message is moved by the client from your Inbox to the Deleted (or Trash) folder. Apple iOS Mail (and I'm guessing your Mac Mail client) has a setup option to move the deleted message to the Deleted Messages folder on the server or the Trash folder on the device.
My guess is that you have Apple Mail set to
save deleted messages to a local Trash folder instead of the
IMAP server Deleted Messages folder. If you delete messages while you are offline (not connected to the Fastmail IMAP server), then the next time you connect your Inbox on the client will synchronize with the Fastmail IMAP server Inbox, which still contains those messages. So the messages were never deleted from the IMAP server and they reappear in your client Inbox.
If that's the problem, changing the deletion action to move the message to the
IMAP server Deleted Messages folder should fix the issue.
Bill