Absolutely horrible – I tried to setup Postfix for my friends to hide Gmail behind their organization's domain.

Configurations were incredibly unfriendly and tedious. Despite all anti-spam measures, Microsoft was blocking their IP according to some private spam blacklists. Then they couldn't send emails to some ancient government server in Guatemala because of encoding incompatibilities.

I ended up moving them to the free MailGun plan. It took me 2 hours instead of 2 days.