A per-mailbox administrative message length limit has been exceeded. The general semantics imply that the individual recipient may not be able to delete material to make room for additional messages.
This status code should be used when the per-mailbox message length limit is less than the general system limit. This is useful only as a persistent transient error.
But if the problem persists (this will be the case for a minority of e-mail users), then “select” the option “My outgoing server(SMTP) requires authentication” as well.
IMPORTANT: Also ensure you are using your full email address as your username.
The destination system specified in the address does not exist or is incapable of accepting mail.