No, it' s not going to be " outgoing NAT" ! That is source NAT and what you need is destination NAT. DNAT is done via VIP and the VIP has to be used in a policy to work.
You could collect all SMTP servers used for a while and see if there is only a handful. If you create more than one VIP you can use a VIP group in just one policy which makes handling much easier.
BTW, I don' t think that convenience is a killer argument for a firewall feature. I am used to work in many different networks with my notebook, and thus I use an IP address switcher tool. It switches address, DNS, gateway, and more if it needs be. I would not expect all networks to be ' convenient' enough so that I didn' t have to budge.
Ede
"Kernel panic: Aiee, killing interrupt handler!"