The largest forum I admin uses PHP sendmail, and the hosting company seems to work hard on keeping up their reputation as we've been with them for 22 years and have maybe had only two or three issues the entire time. The most recent and worst issue was the dreaded problem with Microsoft, where they would not let our mail through their system (notorious issue with them). I contacted the host and it took several months for it to resolve, but it's working again.
For smaller lower-traffic forums, I've used ZeptoMail for a while and it seems OK, but I have to handle bounces manually. Fortunately they are few and far between. Email traffic is surprisingly very low. Even ZeptoMail got bitten by the Microsoft issue but thankfully they resolved it in only a few weeks. I will probably move on but for now, they're doing the job until I find time to look at others.
Amazon SES sucks. I used is successfully for a few years but then, something came up with the account that needed my attention and their menus seem like they are dozens of layers deep and I could never find anything they referred to in their emails, so I finally just gave up on it. I was always safely within the free tier and would gladly have paid (back then) as up until my issues, mail delivery was flawless. But they've made their entire cloud product so convoluted and difficult to use that I have zero desire to use them ever again. (Not to mention not wanting to send Bezos a single penny.)