postfix/dovecot for smtp & imap, spamassassin for spam, use postfixadmin for administering users and i just have squirrelmail set up in case i actually need webmail (i rarely do but boy howdy does it save the day when i DO need it). this setup is very well documented and is, all-in-all, quite hands-off once configured.
you'll have more problems with major companies thinking you're a spammer than you will with the software.
QOTO: Question Others to Teach Ourselves. A STEM-oriented instance.
No hate, No censorship. Be kind, be respectful
We federate with all servers: we don't block any servers.