As of the “Hongsil’s Summer Blast” update, only characters that are Level 36 and higher are able to use the Marketplace, send mail to other characters, and trade with other characters.
Please note: characters below Level 36 can still receive mail from other characters.
If you've already met the level requirement but you are still unable to send mail to other characters, please submit a ticket to Support at support@bladeandsoul.com