X-POST meaning in Internet ?

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.