The security firewall serving Vantage$$s email program catches an impressive number of both irrelevant and illegitimate messages, to the point where employees usually do not even see them. Occasionally, however, an unsolicited message will catch managing partner Carl$$s eye. Because one new message looks to come from a potential new client, he$$s interested but careful, aware that the boutique consulting firm has a very targeted marketing system and clients usually do not appear out of thin air. What type of security concern is causing Carl$$s suspicion?