I had a similar problem, i couldn't send mail to one of my alts, it said that mailbox is locked and i had 0 mail in it. I just had to log it in and the problem was fixed. Perhaps the characters in your factions haven't been logged on for a while so that's causing this... I don't know sorry, hopefully someone provides a better reason
That's something I never knew, and I've played this game for 5+ years. (not all on 1 server)