I agree with Lady Nerium and the original poster. It'd be nice to segment messages to a small group or a drop down list of your league members when you got to write a letter so you can add one name at a time etc. Also a larger inbox to hold more messages would be lovely. Dunno about the other players but being a Captain and dealing with league drama everyday can fill up your inbox quickly and sometimes lose messages before you get a chance to respond or even see who its from.
This would help a great deal but the main factor is being able to send out a message to certain people at lets say soldier rank+. With a maximum of 5 or 10 so for example a member is getting an incoming raid but knows theirs spies of that raider possibly in the league. He can message a group of trusted friends/players to help him out without it being leaked to the raider so you can catch their offense with a defence wall. Bringing in this option will help make general gameplay a lot easier and also helping out new players also