Relatively is itself a relative term. Tens, hundreds, thousands, tens of thousands? You’ll have to quantify at least a little better to attract meaningful answers.
Currently tens but I would like a tool that supports up to tens of thousands to have some space for growth before upgrading.
Good that RichardHesketh asked, because I'd say there's quite a difference between < 100 and > 100000 and it sounds like you look for the latter.
It’s just sending emails, so almost any simple solution with a very cheap server can handle it.
Also going to use SES
Currently looking at the following: listmonk, sendportal, phplist