Thank you! Changed quota and still get same error.
At the same time that sender gets message above, root on the server gets this mail:
The original message was received at Fri, 14 Oct 2011 09:11:48 -0400
with id p9EDBm1g008683
----- The following addresses had permanent fatal errors -----
(reason: 550 5.1.1 <email@example.com>: Recipient address rejected: User unknown in local recipient table)
(expanded from: firstname.lastname@example.org)
----- Transcript of session follows -----
... while talking to [127.0.0.1]:
<<< 550 5.1.1 <email@example.com>: Recipient address rejected: User unknown in local recipient table
550 5.1.1 firstname.lastname@example.org... User unknown
<<< 554 5.5.1 Error: no valid recipients
The "support" mailbox is the address in the Send copy to field and the support entry in /etc/aliases I have commented out.