Do you know if this is a POP, IMAP, or Exchange server?
Some newer Exchange servers won't work on Mail unless you have 10.6 or later.
1and1 is POP or IMAP I believe (it says on their site what the options are). The incoming mail server should be something like imap.1and1.co.uk or mail.1and1.co.uk or pop.1and1.co.uk. The outgoing should be something like mail.1and1.co.uk or smtp.1and1.co.uk.
Hav you tried those?