This is pretty basic: a) By sending a message, the user is consenting to the contents of the message being delivered. b) The user is entitled to request that

This is pretty basic: a) By sending a message, the user is consenting to the contents of the message being delivered. b) The user is entitled to request that you (the messaging service) deletes their details but you have a legitimate business reason for retaining the message details (ie. someone else that you are serving is using them). That said, if your co-founder isn’t more committed than this, then let him go—just make sure you have a legal document stating that he gives up all rights.