Yes, my own mail server for 100-or-so domains with a few hundred users in total, which has been running in its current incarnation since around 2005 (and with 3

Yes, my own mail server for 100-or-so domains with a few hundred users in total, which has been running in its current incarnation since around 2005 (and with 3 or 4 different setups before that, going back all the way to a UUCP node in the early 1990s). The number of domains/users is probably a third of the high-water mark, since many users have migrated to cloud solutions by now, but traffic is still quite significant, with most users having been onboarded long before anything more fancy than Hotmail was a thing. Mail deliverability has, with some minor exceptions, never really been an issue. These days you need SPF, DMARC, and of course a clean outbound IP reputation, but that's all rather manageable -- biggest deal is setting up strict filtering and rate limits on outbound messages, plus ensuring mailbox passwords can't be brute-forced (complexity rules plus blocking abusive IPs). The only real recurring issue is people setting up forwarding to gmail.com/outlook.com/whatever, and the target service then becoming temporarily upset due to all the 'spammy' messages being sent: DMARC helps with that up to a point, but not perfectly, and directing users to reverse the mail replication direction (setting up gmail.com to pull, not my server to push) is a common chore. For the current setup, the biggest headache was finding software with an acceptable webmail and domain setup self-service web interface. Next up was shared access to large mailboxes, Outlook calendar support (which just really can't be done with only CalDAV, even though that should be possible), and (believe it or not) getting 'Drafts' folders to sync across devices.