We had a client who couldn't see his sent emails. The following resolved the issue.
- Go to "Spotlight" on your Mac, top right corner. Type "keychain access". When the pop up screen appears, make sure in the left column you have both the following tabs selected. "Login" & "passwords". From there look through the list in the columns you have and delete EVERY mention of "yahoo" i.e. "smpt.mail.yahoo.com". Basically, if you see a line with Yahoo in it, then get rid of it.
- Go to your Yahoo email account on your browser, before you login deselect the tick box that asks you if you want to stay signed in. After logging in, log out.
- Go to "Mail" on your Mac and open up the prefernce pane. Go to accounts and change the servers for your BT Yahoo email accounts to the following.
Incoming Server - apple.imap.mail.yahoo.com
Outgoing Server - apple.smtp.mail.yahoo.com
4. Restart "Mail". On some of my yahoo email accounts this worked instantaneously for me. On others it took around 5 minutes for the account to start working again. Good Luck to all.