by dasein » 2015-04-22 15:20
I strongly suspect that you aren't going to like this answer, but it is the correct one...
Step 1 is learning enough about SMTP to know which functions/features are SMTP related, and which ones aren't, which probably means in your case that there's a Step 0: learning the basics of client/server architecture.
Until/unless you complete these steps, your questions will be off-the-mark (as yours are, above), and you won't have any meaningful basis for evaluating the answers you receive.
Learning the subtler points of internet email isn't rocket science, but it is unquestionably time-consuming, simply because there's so much to know. So, before you go soliciting recommendations (much less accepting/trying to implement them), you need to get to a place in your head where you can, for instance, spout the difference between an MTA and an MDA without pausing to think about it. Wikipedia is a great starting place.
That said, the cognoscenti's choice in SMTP servers is Postfix: robust, secure, with genuinely exemplary documentation.
One last piece of unsolicited advice: do all your testing/tweaking/migrating on a separate machine dedicated to the task. In particular, do not "upgrade" your existing production server without several weeks' worth of extensive testing of the two systems in parallel.
Afterthought: A good test to see how well your studies are progressing would be to revisit your "wishlist" above periodically. When you are able to look at each item on that list and know instantly whether it is or is not actually related to SMTP at all, then you'll be ready to tackle the "which SMTP server?" question with confidence. Who knows? You might just end up deciding that you don't need to change your MTA at all.
