Welcome to old operating system support!
In today’s edition you’ll find older operating systems don’t work properly with modern and secure and up to date hosting platforms, such as WHM.
In this case, you have to decrease the security on your server to get your clients to log on.
In Exim configuration manager, do this:
Require clients to connect with SSL or issue the STARTTLS command before they are allowed to authenticate with the server. [?]
Disabling this option will significantly decrease the security of the server by allowing the plaintext transmission of authentication credentials.
Next, on the El Capitan user who is out of date, you have to do this:
Port: 587 with SSL disabled
Allow insecure authentication: should be switched on
Allow insecure authentication: should be switched on
Not good, but at least you can go on with your day.
Best idea ever, get the user to upgrade 😉 Or send them a big account.
See also: