Rob, looking at my own 1and1 domains we might be going more complicated than we need to be.
I presume the mail server you want does have not only an external IP but also a domain name associated with it?? If yes then you only need to enter that domain name (no need for the mail prefix) instead of setting up the cname for mail on your own domain.
Also is the mail server set up to recieve emails with your domain?? If not then it'll reject all emails..
For example my
www.raidxtreme.eu website's MX record points directly to raidxtreme.eu and nothing else (the A record is correct for my server IP and as you;re going in on port 25 it'll automatically get routed to the email program)
So your A record should go to **.**.**.** (the IP for your 1and1 host)
and the MX record should go to otherdomain.com (as long as it resolves to the mail servers IP it doesn't matter if it's got the mail prefix or not)
You also need to set the radio button from "use 1and1 mail server" to "use other mail server" (can;t remember the exact text
)