SMTP Error 503
Description: Bad sequence of commands.
What does this mean?
This is also very similar to SMTP Error # 501. The small difference in this, is due to the parameters or options being out of order from what the mail server expected. This is commonly seen when not authenticating the email account.
How to fix SMTP Error 503?
Check to make sure that the email account is authenticating properly (if required which it should!.) If properly authenticating, ensure all MTA configurations are set properly.