Constant outgoing network traffic

[fragility]

Hi,

I've been monitoring the network traffic on our Zimbra (open source) server for a while now, and there has been at least 0.5 Mbit/sec of traffic going out the external network interface (eth0) for the last couple of weeks. This seems to be quite a bit considering how little mail our system processes.

I know that the various Zimbra processes communicate with one another via TCP, but wouldn't the traffic remain on the local system?

Unfortunately I don't have network traffic information going back far enough to compare with the last two weeks.

Any assistance would be appreciated. I can provide Zimbra or network settings if needed.

Is there a way to get stats on outgoing mail, so that I can see the amount being sent, and/or the accounts that are sending it? (In case someone is using their account to send out vast amounts of mail, which is unlikely.)

[fragility]

Alright, I seem to have solved the problem, but I'm not sure of the original cause. I told Zimbra to flush its message queues, and bandwidth usage dropped drastically afterwards. Perhaps a large message had been sitting in the queue for a while, and the server kept retrying it? Where can I go to detect things like this? (Other than watching my bandwidth usage?)