SimpleMail does work for me on MorphOS still (at least the OpenSSL version), and I guess it'd still work on Amiga too, but the problem is if email providers start to require "stronger" authentications. I guess the current SimpleMail will stop working soon with GMail when Google starts to require OAuth2, but it probably continues to work with other providers for now.
But if someone still develops SimpleMail, I don't see it being that difficult to update it with OAuth2 support, and AmiSSL v4 is still in development, so why not?
IIRC there are some AmiCygnix email clients for OS4, which might become solutions when they mature a bit, although they're not that "native" then, but a solution anyway. MorphOS is in the best situation, because there is the native
Iris email client that supports Everything (tm) currently (Oauth2, IMAP, UTF-8, HTML5, etc), but unfortunately it might be a bit difficult to port it on Amiga in the current situation...