I use mailboxorg its acceptable.

The webmail has io.ox in the URL so I assume it is hosted ox (www.ox.io of softwares) which is SIGNIFICANTLY worse performance and usability wise compared to the Fastmail webmail -- I don`t use SMTP/IMAP so I do not know what that is like. Routinely on restricted networks that only allow HTTP/HTTPS so it is a webmail only experience opinionated in mine.