I run a VPS through InterServer, pay about 20 USD per month but that’s for 4 cores and 8 GB RAM which of course is overkill for mail, but I use it for a lot more. Like Lemmy.
You can absolutely do it with 1 Core 2 GB and that’s more like 5 USD per month.
I use iRedMail which is open source and they have good documentation. But fair warning that it’s quite a bit of work self hosting mail and getting your mail to not end up in peoples spam folder. Still very much worth it though.