I'd say the majority of professionals are using postfix with lots of spam scanning add-ons as their heavy-hitter MTA of choice. There are still sendmail die-hards and it works, but postfix came out of some needs that sendmail wasn't providing in the 90s. They eventually caught up and realized they were getting eaten alive. So both work. Once you learn one you'll tend to be a fan so whatever.
For user access to mail, I'm partial to dovecot these days.
I recommend starting directly with Maildir storage format and bypassing the stupidity and poor design of mbox files.
By the way, dovecot supports its own proprietary on-disk format called dbox. Here's why I don't recommend it for servers that don't need it: It's non-standard. Let 's say a huge critical emergency security warning is issued for dovecot without an available patch right away. (Rare, but we are talking about why to use standards here.) You can uninstall and replace dovecot in about 15 minutes to something else that understands Maildir format. That's my take on it. Everything understands Maildir format these days.
As someone who's run high-volume e-mail servers for a couple of decades, I'll warn you that once spammers "find" the domain, in an increasingly hostile Internet environment, you'll spend far more time messing with the spam fighting software and tweaking it for your users than the MTA or the Delivery mechanisms. They'll generally just run.
I got tired of it and let other professionals get paid to waste their days of their lives on it for dollars. You can automate a lot of it, implement nice things like optional TLS encryption and authentication/identification of remote mail servers, teergrubing the idiots attempting to kill the server with bulk spam and drive your bandwidth costs up, flat out firewalling Asian and other foreign IP space if none of your users expect any mail from overseas servers (a constant source of attack), etc... And spam will still get through.
Think long and hard if you want an unpaid e-mail admin job for either just yourself or a group of users who WILL whine during times when you want to do other things that are more interesting.
I ran my personal mail servers with free accounts for family and friends for a decade or so until GMail was widely available and made e-mail a free commodity. Then I politely told users they had six months to find another "provider". At that point I tracked that I was spending an hour or two a week messing with fighting spammers even with a well-configured mail server running piles of anti-spam and anti-virus add-ons that had been built up over that decade. I realized I wanted my 104 hours a year back.
I moved to paid service with fastmail.fm who at the time was well known for doing one thing, and one thing only. E-mail. Their developers were deeply involved in the bug fixes and extension of the IMAP server they were using as well as being in the forefront of anti-spam and extremely high volume MTA work on redundant distributed mail platforms. They were later bought by Opera, the browser folks and I lost touch with their work as I had migrated to Apple's MobileMe platform.
One note on IMAP. Use it. POP3 is stupid in the multi-device world. Server stores the mail and all devices "see" the current state no matter which device is manipulating, deleting, sending, whatever. When I moved these mailboxes I simply used a popular IMAP client (Thunderbird) to copy a decade worth of saved mail from one server to another. Drag and drop and come back a day later. Nearly flawless and I can full text search over ten years of saved mail for things I know are I'm there but can't find.
Anyway there's some "old-skool mail dude" notes. Probably easier to pay someone a pittance and let them deal with the headaches unless you have a dire need to run your own mail server.
Whatever you do, learn how not to build open-relays and back scatter spam launching points or you'll raise the ire of other mail server admins who'll just report ya into the various DNS-based real-time spam blocking " databases" and then good luck getting off of them.
Most distros ship with a local-only MTA config nowadays for a reason, too many folks clueless about mail servers and allowing their machines to be attack vectors for everyone else in the swimming/cesspool of Internet e-mail.
