The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account 1 (2013-04-30T16:36:52). Exception Message: Cannot send mails to mail server. (Mailbox unavailable. The server response was: 5.7.1 Client does not

Interesting db mail issue. If i click send test mail from sql mgmt studio, it works fine, but when i execute a SP to send a mail, it fails. One thing I noticed is that the "LastModified" column in the mail log shows the domain account when a test mail was sent from mgmt studio, but show "sa" when the SP was used to send mail. 

This is from SQL 2012, did not see this in sql 2008, looks like 2008 always used the service account.  any ideas ?

Get this error:

The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account 1 (2013-04-30T16:36:52). Exception Message: Cannot send mails to mail server. (Mailbox unavailable. The server response was: 5.7.1 Client does not

Thanks.

Ranga