And the email account settings?
This is what BT say they should be:
IMAP
Incoming Mail Server: mail.btinternet.com
Port: 993 (this should be automatically populated if you tick the SSL Encryption)
SSL Encryption: Enabled (but not STARTTLS)
Username: your email address including the @btinternet or @btopenworld.com part
Password: your btinternet password
Please do NOT enable "secure password authentication" (SPA). It's not needed when you use SSL encryption and won't work.
If your IMAP client supports it, we recommend you also set up the following folders:
Trash
Sent Items
Drafts
SMTP
Outgoing Mail Server: mail.btinternet.com
Port: 465 (this may not be automatically populated on selection of SSL, so you'll need to check this)
SSL Encryption: Enabled (but not STARTTLS)
Authentication: PLAIN
Username: your email address including the @btinternet or @btopenworld.com part
Password: your btinternet password
iCloud Mail Account
Go to System Preferences>iCloud: switch Mail off, wait a moment, switch it back on again.