Setting MailKeyIPVerification to false just means that the UCS will not check that the IP address being used to connect to it is the same as the one that was used to login to the worldserver.
In theory, without the IP verification, someone could brute force a connection to a characters in-game mailbox (if they knew the character name) by trying every possible key value. This would entail writing a client program to talk to the UCS especially for hacking ... it's not something that a casual user could attempt.
If it's just a private server, and/or you're not sending any top-secret information by in-game mail, there is really nothing to worry about by setting it to false.
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