There is a limit of how many messages you can send to people that aren't on your contacts list. The limit is renewed every 30 days but not always at the beginning of the month. During this period you can send 2 messages to people not in your contacts. After sending a message, you can conduct an uninterrupted conversation with the recipient of that message without exceeding the limit.
The limit is intended to reduce the activity of the so-called spammers who send trade information this way.