Have doubts about email services. Can I use this with a server behind NAT on a Dynamic DNS ?

I think Store/Forward would be enougth, but in any case, if that does not work can I switch to Full Email Server ?

That is exactly how I'm using it. I have a email server behind a nat at home.
I use it to relay the incoming mail to a non standard port other than 25 as most ISP's block that port for mail delivery. That is the only reason I use the store and forward service, to get around that block.