I actually just discovered all of the errors were going into my spam folder... not something I expected. I guess just for future reference by others trying to track it down, this is the the header information I got received:
The original message was received at Sun, 18 Oct 2009 07:25:15 -0700
from localhost [127.0.0.1]
----- The following addresses had permanent fatal errors -----
(reason: 553 sorry, your mail was administratively denied. (#5.7.1))
----- Transcript of session follows -----
... while talking to relay-hosting.secureserver.net.:
>>> MAIL From:<!email@example.com> SIZE=2452
<<< 553 sorry, your mail was administratively denied. (#5.7.1)
501 5.6.0 Data format error
Final-Recipient: RFC822; !firstname.lastname@example.org
Diagnostic-Code: SMTP; 553 sorry, your mail was administratively denied. (#5.7.1)
Last-Attempt-Date: Sun, 18 Oct 2009 07:25:15 -0700
"501 5.6.0 Data format error " made me think plain text mode might solve the solution, but no dice.
I fixed it by setting the from address to email@example.com after talking to a friend who is a sysadmin. He advised me this might be an SPF issue, and to "Use an outbound mailserver that doesn't require spf...."