If you follow any USENET newsgroups, sooner or later, you'll find the need to post your article to more than one group. This is called cross-posting. Generally, you want to keep this to a minimum. All mail2news gateways put limits on the number of newsgroups allowed. Some permit five while some only permit three. Simply testing is as good as any way to determine how many groups a given gateway will allow. If you exceed the permitted number, your message will be discarded. Certainly, if you keep your groups to a maximum of three, you'll always be safe.
On rare occassion you may need to post to more than the allowable amount. In this case, you can send your message more than once, changing groups between sends. This method is known as multi-posting, rather than cross-posting.