I have fond memories rewriting sendmail.cf and mc files on BSD. Ran servers on pc hardware and mirrored hdd drives; they pushed mail like nobody's business.
If the Exchange box fell over at the weekend, sendmail would sit there patiently waiting (screaming DEFERRED silently to log) with tens of thousands of mails until it came alive again, and push them out in minutes, faster than Exchange could receive (I remember spamming #sendmail -q and you could watch the disk activity freak out on exchange ) .
Confident sendmail is best mta ever. Do the big players use something else these days?
-- Edit to add an ai poem for my favourite mta -
Sendmail is not a simple mail app It's a mighty mail transfer agent that can adapt It can process SMTP commands and deliver mails To any destination that it can access
Sendmail is flexible and extendable It can be modified to fit any goal or role It can sort, redirect, transform, and control mails With its intricate rulesets and macro code
Sendmail is better than MS exchange It's more secure, dependable, and swift It's the default MTA for many Linux systems And it's free and open source, unlike its rival's gift