You can't send mail to folks if there are spaces in their gamer tags. It won't even work if you use the send mail feature directly from your in-game guild list or friends list. I've included the entry from the known issues post below:
[New 6/15] Unable to send mail: We've seen reports that you sometimes cannot send or receive mail. This appears to be caused by spaces in a player's Xbox Gamertag. We're actively working on a fix for this.