I have a windows application that sends emails through our zimbra server. It attempts to send 200 emails, however when it gets to email number 121, an error is returned:

4.7.0 server.domain.net Error: too many errors

and the emails are not sent on from zimbra to their destination.

If I code the program to send 100 emails and then pause 5 minutes and send another 100, the emails all go through.

If I code it to send 100 emails, pause a few seconds and then send another 100, I get the error at the 20th email of the second batch and the emails don't go through.

Is there a setting in zimbra (or in postfix?) that controls this? And if so, how would I change this?
I am a programmer and not familiar with zimbra administration, so I'd appreciate if any suggestions are spelled out for a newbie.
Thanks in advance,
Jim