Not only can you mail to characters of the opposite faction, you can play with them too!
With the exception of Sanctum, Meridian, and Guardian/Defiant Outposts throughout the original zones you can freely group and guild with any player on PvE shards. More details: http://community.riftgame.com/en/201...e-gap-in-1-10/
James "Elrar" Nichols
Former Community Manager
Trion Worlds, Inc.