https://support.bladeandsoul.com/hc/en-us/articles/207420056-Why-Can-t-I-Send-Mail-To-Other-Players-
"Players who have not made any real-money purchases since August 18th, 2015, will not be able to send any outgoing mail to other players.
However, you can send mail to your alternate characters on the same account without restriction.
You can permanently remove this mail restriction by making a real-money purchase, such as buying NCoin."
When I go to mailbox it just says "You are required to charge your NCoin at least once to send mail."
http://imgur.com/W