Alright, I didn't quite understand what you said about personA to personB at first. Sorry about that.
To clear this up, this can be done in queries, other chats, and/or server(s). As long as your bot is there. The person must type !msg UsersNameYouAreSendingAMessageTo YourMessageHere. Then it will be sent to the bot and kept there till the person retrieves the message by typing !msg.
I like it. Good job.