Maddy instead of Postfix because Postfix configuration gets quite messy with complex virtual users logic. I was trying to refactor it, trying to devise some sort of DSL to make things more bearable - and found myself wishing there'd be something more straightforward. Maddy was a good fit (and it has a modern easy-to-understand codebase that I can hack on it if I need to). It's much smaller and simpler than Haraka, but still has all the features I need, and looks secure (I swept through the source code and found nothing obvious).
Dovecot is a very fine piece of software. I was considering Wildduck, but haven't figured out a contingency plan if I'm going to dislike it in the long run. As long as all my mail is a Maildir (with some extra indexes I can just ditch), I have no worries - I'll always be able to read it. So I've just set up dsync for the failover (in case one of my servers has a network or power outage) and can't be happier.
Once (somewhere in early 2010s) I had a backscatter issue that put me on a couple DNSBLs, fixed that the same hour I've learned of the issue, filled out some forms on RBLs I was listed on, got removed in a couple days.
Delivery-wise, I've had some issues with Outlook (to an extent I've almost set up an "embassy" there, tricking the system into thinking that my domain is hosted with them and using their own MSA for my outgoing mail to Outlook-hosted domains - proof-of-concept had worked, but I haven't really bothered to implement it in practice as they had suddenly started to receive my mail without issues, and this whole idea is a dirty hack), but otherwise email delivers fine. A couple times Gmail put my emails into spam, but mostly everything worked as expected. Not that I send many emails, maybe a couple dozen a year - it's mostly receiving.
I'd say the main problem is lack of decent mail clients.