"Mail can't verify the identity of imap.mail.att.net. The certificate for the server is invalid"
I clicked "Show Certificate" and checked the box "Always trust" (despite my misgivings) but the next time I tried to retrieve my mail, I had to go through the whole process again.
I spent an hour on the phone with ATT yesterday and he couldn't figure it out. I spent much more time on Google, trying suggestions, but I haven't had much success either.
I started with the obvious, deleting preferences, mailboxes, accounts, passwords, and certificates, and the usual maintenance checks. I even tried downloading a free SSL certificate from Commodo but I don't think it worked because I can't tell if it downloaded.
OS X 10.11.6
iMac 21.5", Mid 2011
2.8 GHz Intel Core i7, 24 GB
AMD Radeon HD 6770M
Using Apple computers since 1980