A bug involving private messaging was just reported and quickly fixed. Unfortunately, this bug had been present since the server move at the end of March. It would have probably affected a small percentage of messages, so that's probably why it took so long to catch.

The bug would occur if an apostrophe was present in the SUBJECT line of your message. For example, a subject line of "How's it going?" would cause the bug to occur. The email notification would be sent to the recipient, but the message would not appear in the recipient's Inbox nor the sender's Outbox.

If you think this may have occurred with a message you sent, please check your history of Sent messages. If you can't find the receipt of your message, you will need to resend since your recipient never received it and may not have been notified if their email address wasn't current. They also might have thought you retracted the message even though it's currently not possible with the private messaging system.