You wrote "to another group of users using the same list" and I am not sure I follow exactly what you mean.  My thinking is "a list" is equal to "a group of users" in the usual case - so sending to 'the same list' means you would be reaching the same users (unless maybe TOPICS are involved).

Anyway, to what I think you're trying to discover - no explicit restriction exists (beyond bandwidth and processor capacity).  You may send as much as you like as quickly as you like.  Incoming traffic is placed into a queue and processed as fast as the system is able to handle it.  The incoming mail receiver on the LISTSERV system is designed for multiple concurrent inbound sessions and often has traffic arriving simultaneously from more than one sender.

The LISTSERV process is single-threaded (generally) - one incoming message is found, distribution is done, then it looks for another message in the queue.  But that doesn't really have anything to do with the process of email being addressed to lists and sent from hither and thither.  The receiving of messages into the queue, and the sending of messages to list subscribers, are separate processes that operate independently.

As for the mail-handler that LISTSERV drops the traffic onto for delivery, typically no specific restriction exists there either. A mailserver is designed to do more than one thing at a time and generally does not really care how often or how many pieces of mail arrive.

>>> "Harpe, Shawn" <[log in to unmask]> 11/18/08 2:39 PM >>>
Question: How soon after a message has been sent to a listserv list and it is being processed (messages have starting being sent to the subscribers) can one start to send to another group of users using the same list?

Thanks,
Ron