Yes you will want to create an access-list and apply it to the outside interface:
access-list out_in extended permit tcp any host 203.98.225.9 eq smtp
access-group out_in in interface outside

Also, if the email server is located off of the inside interface instead of one of the other interfaces, add this to the existing access-list:
access-list in_out extended permit tcp host 10.10.10.16 any eq smtp