GUI Error - Email SMTP

ERROR

421

<domain> Service not available, closing transmission channel

500

Syntax error, command unrecognised

501

Syntax error in parameters or arguments

502

Command not implemented

503

Bad sequence of commands

504

Command parameter not implemented

521

<domain> does not accept mail

530

Access denied

535

Email Username and Password not accepted

550

Requested action not taken: mailbox unavailable

551

User not local; please try <forward-path>

552

Requested mail action aborted: exceeded storage allocation

553

Requested action not taken: mailbox name not allowed

554

Message refused / Transaction failed