Cross-posted messages are email messages that are sent to more than one mailing list on a server.
If a person is a member of several mailing lists, they might choose to send an announcement to all the mailing lists they belong to about something they feel is important. You, as the list
administrator, may wish to prevent this. Cross-posting is a common technique of "spammers".
By default, cross posting is forbidden, but it can be allowed by changing the list settings for a mailing list.
A common use for cross-posting is when a useful announcement does need to be sent to many mailing lists. Support, for instance, that you were the list administrator and wanted to tell your many users
that you would disconnecting your Lyris server for a week while you moved your house.
In this case, you would want to allow the cross posting. However, if a person is a member of several of your mailing lists, they would receive multiple copies of the announcement: one for each
mailing list they are on.
There is a simple solution to this problem: with a cross posted message, Lyris can be told to remove the members that appear on multiple mailing lists, so that each person receives exactly one copy
of your announcement. This is set in the list settings for each particular mailing lists,
The postings from List administrators are always allowed to be crossposting (in other words, this setting has no effect on list admin postings).