A few days ago, my iMac lost power briefly. It re-started OK, and everything seems fine, except Mail. The power loss may be irrelevant, but I can't see anything else that's changed.
These are the problems:
1. The last few emails received before the power loss lost their content. I'd just read them, content was there, then after the power loss, it had vanished.
2. The Outbox has vanished. I seem to be able to send emails (I've tested sending to another address and the emails are reaching it) but there's no Outbox and no Mail activity progress bar while I'm sending.
3. The sent emails in 1. above don't show in the Sent folder.
4. I'm unable to download any emails from my Plusnet account. The Mail activity progress bar says that there are incoming emails, but nothing appears.
5. The number of emails being downloaded changes each time I do Get mail, as though the emails on the mail servers have lost their "downloaded" marker. (I had set the Mail application to allow messages to be deleted from the servers a month after downloading, but I've just turned this off temporarily to avoid losing everything if this problem persists.)
6. Bizarrely, this afternoon two emails have appeared in my in-box, from the dozens that have come into my account since the fault appeared.
I got a system message to say that the Mail index had been corrupted. I rebuild it, and now I can see the mail from the time following the power failure.
(However, comparing the webmail inbox with mail downloaded to Mac Mail, I found that the three whose content was removed in Mac Mail by the power failure had vanished altogether from Mac Mail, so I forwarded them to myself in webmail to reinstate them on the Mac.)
The Outgoing message Activity bar has reappeared.
I've lost a few sent messages - ie Mac Mail failed to file them in the Sent folder after the corruption occurred.
The Outbox issue was a non-issue. I've tested it by taking Mail offline then trying to send a message, and of course it appears then to store the message.