Because 'company.com' represents the whole domain, not a single computer in that domain, you do not need to enter it as a permanent host with a unique IP address. You just want to assign a mail exchanger to it. When e-mail is to be delivered to 'user@company.com', the sending mailserver will query for the mail exchanger of 'company.com' and get the address of 'mail.company.com.
Here is how to accomplish this:
- In MacDNS, create a permanent host for the server itself (if you have not already done so), giving it whatever name you like, such as "mail.company.com". You may want to make some aliases to it, if it runs other services as well, such as "ftp.company.com" and/or "www.company.com". Also add a mail exchanger for the server, which would just point back to itself (example: 10 mail.company.com)
- In MacDNS, create an 'MX-only host' record, for host 'company.com'. Type in the appropriate numeric preference and hostname in Mail Exchanger field, such as "10 mail.company.com".
- In AIMS, under 'Server:Preferences" menu in "Server names", add both of these domains (for example, 'company.com' <default> and 'mail.company.com').