That's the alert message to the user. The email that was infected had an attachment, which the mail server would be expecting. If WebShield just deleted the attachment, it would be a corrupted mail, so WebShield has to replace it with a .TXT file. I believe you can disable this (please feel free to correct me on this, as I haven't played with WebShield SMTP all that much)
HTH, AVChap
... my $1 worth of advise, 2cents isn't enough due to inflation