For anybody interested in more choices for volume-based providers like PurelyMail (with tiers based on storage and emails sent/received but who otherwise allow unlimited domains/mailboxes/aliases) there's also MXRoute (US) and Migadu (Swiss/EU).
These providers don't usually make sense for a single mailbox (although some of them have a low entry tier for this purpose) but can be extremely cost-efficient if you need 2 or more mailboxes/domains.