Yes, there are a couple ways in which posting frequency is limited. One limit is the number of messages posted to any single group within a short period of time. We also limit the number of messages a single user or computer can post via our website in a short period of time.
If you reach the first limit, your posting ability will be frozen and you'll be notified via email. The email will look something like this message:
Your message could not be posted to the [groupname] group because there have been too many messages posted to the group recently.
If you run into the second limit, you'll see a notification directly on the Groups interface.
Please note that we're unable to provide the exact numbers that trigger these posting limits as they are something we manipulate for spam fighting purposes.