If you have an Apple Mail client (MacOS, iPhone, whatever) that recently stopped sending email to some servers, the reason is because Apple doesn’t play nicely with other products. There is a solution…

SMTP conversations begin with the client saying HELO (or EHLO) and identifying their hostname or IP address. Clients that send garbage or the server’s IP address are spamming tools. Therefore, watching for bad behavior here is a good anti-spam measure.

Apple changed their client behavior sometime in the last few months, and it shows up irregularly as people update their software.

If you are running a Postfix server, the setting you need is

smtpd_helo_restrictions =
        check_helo_access 

Find the postmap database that check_helo_access is using and add a line like this:

smtpclient.apple OK

then re-map it and things should start to work.