Ok,
So I been hunting down why my site is posting 15 as content..
I find out that when you have the mailbox plugin set for mail only and you send no subject and content and you submit the message it goes true..
You get a subject as 1 in the subject line, and as content a 15..
It only happens with mail only is set in setting not mixed mail and chats..
Now here is the kicker, the message posts only if you account isnt the admin account ..
If you want to test here in Oxwall site, send a message and it will post..
So the bug exists but if you try as admin it won't happen..
You will get an error saying that a subject is needed..
Please if you check please make sure you maibox setting is only mails and you use a regular account..
Also , don't set debug mode in your config.php the bug won't show ...