Email Sending Failed - SMTP Error: data not accepted. - WHMCS Not sending emails due to this error

Email Sending Failed - SMTP Error: data not accepted. - WHMCS Not sending emails due to this error

I have been trying to figure out a fix for about a week now and I haven't found one on my own so I am going to ask for help on here. 

After checking all the settings and even resetting my password for the email used for WHMCS it still says:


Email Sending Failed - SMTP Error: data not accepted. 

I have no clue how to fix it at this point. Any insight would be lovely.