What are my options? There is Postfix and Sendmail and qmail and dovecot and and and. It is confusing.
Basic needs, run on Ubuntu. Maybe extensible if I want to write my own plugins for the mail server in the future.
Of the many things that can go wrong, here are just two: 1. You can get a recycled IP address from your hosting provider that has already been added to various spam blacklists. 2. You can get the mail server program up and running, but be rejected by all the major (and some minor) email providers for not supplying the right email headers, and/or, not having the right DNS entries that they want you to have.
>>I think the best way to implement it quickly is to use a mail server
Unless your idea absolutely requires a mail server to function, I would recommend searching for another means of implementation, if only to save yourself the time and learning-curve of modern email systems.
On the other hand, I'm actually very glad that I took the time and energy to set up my own mail server, since it was an interesting, but frustrating, learning experience. But on the other other hand, it doesn't inspire a lot of confidence when I have to call a client and give them a different address because my mail server is borked.
Long story short, go with exim and dovecot. And be prepared for it to take you 4x-10x longer than you think it will.