In toolbar go to Mail/Preferences
In Accounts tab 

incoming mail server = imap.astro.ex.ac.uk
username and password from J. Rowe
use SSL
port 993
password authentication

outgoing mail server = mail.ex.ac.uk
with university username and password (e.g. rk222, not you@astro.ex.ac.uk)
use SSL
port 25

Can use GPG and GPGMail for encryption and signing if you wish.