That is a hard question to answer. Almost all of my domains are hosted with a hosting company (as expected). Email is just a part of the hosting platform, but my total bill is about $15/month. This includes hosting, auto-replies, and email. I have been with Dreamhost for years, and don’t see me changing. However, if cost is the absolute most important thing, consider BlueHost. When I checked (6-1-2022), it was $3/month, and only provided for 1 site. I am not sure about how many email addresses you can have, but you would need to consult with them for your needs. To have an assurance of unlimited, I will go with Dreamhost.