If you are getting this error message on some of your Email forms via PHP, there is a chance your script is not working with secure authentication via SMTP.
SMTP AUTH is required for message submission on port 587
That’s one of the ways to prevent spam issues on your server.
However, if you can’t update your code and need a fix quickly, on cPanel servers you can enable relay through IMAP or POP if they have authenticated in the last hour, it’s called ‘POP-before-SMTP’. You can enable this by moving to WHM – Tweak Settings
Then set this option to On:
Allow users to relay mail if they use an IP address through which someone has validated an IMAP or POP3 login within the last hour (Pop-before-SMTP)
That should get rid of that error message.
Remember this should be used as a temporary solution, your scripts or your client’s scripts should always authenticate via SMTP in order to send messages. Having this option always ON may impact negatively in your email security.