Using an Apple Mac with yahoo, btinternet or btopenworld email accounts

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.
Use:
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.