I do not know how to change the smtp port for SME but you shouldn't need to do that.
You didn't give a lot of description but here is what I can assume.
You put your https:// section of the site on the outside of the firewall because it's secure.
You put your http:// section of the site on the insode of the firewall because it's not secure.
I assume that you want to do the same thing with email. Have secure mail on the sme box and unsecure mail on the suse box.
If you're going to be using pop/smtp, which it appears that you want to, then simply setup SME as your email server. I'm not a big fan of mixing ssl with pop/smtp. If you're wanting to use webmail with http:// and https://, SME can do both at the same time.
Hope this answers your questions, if not, explain with more detail.