I understand the need for no open relays, but is there a selective way to disable this?
Not in a way which would ensure you are not an open relay. It is possible if your phones have fixed IP addresses, but I doubt that they do.
SMTP auth is the other way I might be able to address this. I tried using SMTP AUTH LOGIN PLAIN but SME rejected it. I searched the forum and there is a post about it here, but no resolution
SMTP AUTH is enabled by default over SMTP/SSL (port 465). It is not enabled over non-SSL SMTP (port 25) as it passes usernames and passwords in a lightly encoded/reversible format.
If your phone supports SMTP/SSL, it should just work. If you really must use SMTP AUTH over plain SMTP, you can do this:
config setprop smtpd Authentication enabled
signal-event email-update
WARNING: Your usernames and passwords will be open to username/password sniffing. This is a serious security risk.
And for the record, we don't support CRAM-MD5 AUTH as that would require us to store copies of the password in clear text on the server, which we also don't want to do.