Migadu mail is very cheap, been using them for almost a year now with zero interruption or issues. Unlimited custom domains, unlimited aliases, etc for $20/yr
Currently using the middle tier and have been very happy with it.
Note that codified aliases are limited, but you can create a catch-all account. I have not needed to codify aliases to send "as" one of those accounts.
you can just add +myalias to most email services and get it delivered automatically. E.g. first.second+github@gmail.com delivers to firstsecond@gmail.com
Not just "can," they do strip the + part. Especially the sketchy ones. Or they barf. I tried this for a while and had a 30% success rate. It was dismal.
PretzelBox.cc (my project) does this along with a blog, a file storage area, and hosting.
Final pricing is still TBD but if you join the beta list, we are handing out huge discounts which’ll bring your effective cost between $5-15/month for everything.