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Crossposting is the act of posting the same message to multiple information channels; forums, mailing lists, or newsgroups. This is distinct from multiposting, which is the posting of separate identical messages, individually, to each channel, (a forum, a newsgroup, an email list, or topic area). Enforcement actions against crossposting individuals vary from simple admonishments up to total lifetime bans. In some cases, on email lists and forums, an individual is put under a Stealth Ban where their posts are distributed back to them as if they were being distributed normally, but the rest of the subscribers are not sent the messages. This is easily detected if the Stealthed individual has two different, and totally non-associated identities in the channel, such that the non-stealthed identity will see a different set of messages, lacking the posts of the stealthed individual, in their view of the channel. 
Crossposting to groups that are irrelevant to the message posted could be considered spamming. Moreover, excessive crossposting is generally considered bad form because it multiplies traffic without adding any new content. In the extreme case, if all messages were crossposted to every email list or forum, then every email list or group would look exactly the same. A crossposter can minimize this problem by specifying that all responses be directed to a single group.
In Usenet, the destination newsgroup(s) for a particular message is indicated in the "
Newsgroups:" line. Most commonly, just one newsgroup is specified. For example;
However it is possible to specify that the message is intended for more than one newsgroup.
In this case, the message will be visible both in the
comp.simulation newsgroups. Despite appearing in two separate places, only one message has been posted. This has several advantages.
Followup-To:" line a poster can indicate that answers to his post should be posted to another set of newsgroups than those listed by the "
Crossposting is usually practiced when material is relevant and of interest to the readers of more than one newsgroup. However sometimes it is used maliciously to begin a thread between newsgroups whose readers are likely to have violently differing opinions, in the hope of provoking a conflict. This is a form of trolling.
Crossposting to more than a small number of newsgroups is likely to be counterproductive. A commonly suggested limit is three newsgroups.
Some NNTP servers drop posts which are crossposted to more than a set number of newsgroups, especially if no "
Followup-To:" line exists.