I mostly did it as a part of devops/sysadmin work for various companies I worked for and companies I contracted for. A mix of user email and B2C/B2B email. A lot of time spent trying to keep off of spam lists or unflag ourselves with providers, follow specific mail provider's requirements for sending their user's email, that kind of thing. Back then there were multiple standards for verifying your egress mail servers were legitimate and I imagine things have settled a lot. And of course contending with sales people that got too aggressive with marketing from their company mailbox or marketers not following guidelines for solicited (or unsolicited mail).

There's a lot I would have done differently if given the chance and that's not even knowing what the field looks like today.