What do the SMTP log files report in regards to the SMTP transaction with the Gmail server? From the bounce message details it looks like there problem is the transmitting of the message data. Are you sending from a client machine using a third a party email client? I.e: Outlook? Does the client machine have Antivirus email scanning options enabled? If so try and disable them and then try to resend. If you the client routes via firewall/proxy then this may also be the problem where the connection maybe dropped by the firewall/proxy because of the data being transmitted.
Ian, the log files didn't reveal much. The SMTP protocol traffic looks alright, up until the last two lines, which appear to unexpectedly quit, as shown below.
DATA DATA 354 Start mail input; end with <CRLF>.<CRLF> 46 6
QUIT QUIT 221 Service closing transmission channel 42 6
Here's the strange thing - I rebooted the server for some OS updates, and it released the message. Weird. I'm not sure what was blocking it but the reboot appears to have corrected it.
Thanks for the help.