These instructions were written for OSX 10.5 but will likely work for most recent versions.
Name: ###NAME###
Email: ###USERNAME###
Password: ###PASSWORD###
You'll be given the message "unable to verify account name or password" and you can then add some additional information. Fill out the following before clicking the "Sign In" button again:
Username: ###USERNAME###
Password: ###PASSWORD###
Account Type: IMAP
Incoming Mail Server: hosting.opendoorinternet.co.uk
Outgoing Mail Server: hosting.opendoorinternet.co.uk