Hi Martin,
This is somewhat ironic... if you are running Data ONTAP 8.1 or later, you can specify a non-standard port, already!
However, it is not checked for "correctness", so you need to be careful with the syntax.
Basically, you can append the mailhost name or address with ":<port#>".
cDOT:
system node autosupport modify -mail-hosts my.mail.server.com:2525
7-Mode:
options autosupport.mailhost my.mail.server.com:2525
The burt I logged will drive improved input checking and documentation for this.
Let me know how it works for you...
-Andris