What I did in this scenario is allowed traffic from the mail server and then denied traffic from any other machine. I use ports 25, 26 and 465 because they are all commmon SMTP type ports. In case you are not familiar with ACL's the 'block_smtp' is just the name of the ACL and has no affect on how the ACL runs. You could change this if you want, but if you do then change it on every line. Also changing the order of how these are applied will change how the ACL operates. If you need to remove any items simply put a 'no' in front of them.
access-list block_smtp permit tcp any host x.x.x.x eq smtp
access-list block_smtp permit tcp any host x.x.x.x eq 26
access-list block_smtp permit tcp any host x.x.x.x eq 465
access-list block_smtp deny tcp any any eq smtp
access-list block_smtp deny tcp any any eq 26
access-list block_smtp deny tcp any any eq 465
access-group block_smtp in interface inside
Bill
edited to fix cut and paste error and add additional instructions.