To be honest I wonder if the RMT excuse is just that... an excuse, and if either the servers somehow will explode if we mailed each other more frequently or they feel the ability to mail alts would hurt retainer sales.

In 8 years playing WoW for example, where you can mail alts, guild members, random players, whoever you like... I've never once been mailed by an RMT seller or seen any issues arise.