You are sending messages directly out from Outlook through the Symantec Messaging Gateway (SMG) and using the sender-throttling feature. When sending to a number of recipients less than the configured limit, you notice that sender-throttling still blocks the message.
“Sending reported error (0x800CCC78) : 'Unable to send the message. Please verify the e-mail address in your account properties. The server responded: 421 4.7.1 Recipient quota exceeded'”
If there is an rset mid transaction requiring the re-sending of recipient data, SMG will count this as a secondary count toward the sender-throttling.
Either increase the sender quota, or resolve the rset that is causing the re-transmission of data.