First, there is a weird issue when the advertiser selected not to get messaging copies to their email responses when the email is within the Contact field, that caused the error similar to what you described. It has been fixed and will go out in the update later tonight.
If there is still a problem, please send me a note or post here 1) the listing # of the ad and 2) if it is a contact address (the [@Email] button under the Contact information) or the site messaging response box that follows the Contact information. Both let you track your response in the messaging Outbox but work differently for the advertiser.
