...
Mail problem: SCHEDULER-320 could not send mail (is 550 5.7.1 Unable to relay for ...)
The SMTP server fully REJECTs rejects the mail address xxx@yyyyyy.com with the inforamtion: 550 5.7.1 Unable to relay for xxx@yyyyyy.com) and DELETEs deletes mail. In addition, mails are not sent out to other recipients.
...
If this does not work then ask your SMTP support why a mail with this address was DELETED deleted from the server. You can find more information about why a server deletes a mail and give out this message on http://www.eudora.com/techsupport/kb/1593hq.html.