Do you mean he/she does this on a regular basis, or has done it more than once? Is the message directed towards you in particular or to anyone on the board?
The only thing I can think of is the person realised it was a mistake to post it to a pubic board and so quickly deleted it, for reasons already given. Or they meant to send you the email address but mistakenly posted it to you on a public board.
If they do this often then they could be like me... sometimes I'll make the same mistake several times before I stop doing it.
Maybe it was posted to get a certain person's attention, it did, and was deleted because that job was completed.
Is the person who posted it someone you're sure exists? Are you the only person who has retained any memory of him/her? If so, I wouldn't announce that, or THEY will come and erase your memory. (Sending you tinfoil to make a nice hat, just in case.)