How do I fix 550 SMTP AUTH for message submission on port 587?

This error implies that, unless “POP before SMTP” is enabled, customers will not be able to send email without using SSL via ports 587 or 25. So, to immediately fix the error for the users in the server, the option is to enable ‘POP before SMTP’. To do that, go to ‘WHM –> Tweak Settings‘.