[thelist] Spurious characters in email subject header

[Les Lytollis]

Hi All

I have an ordering application that allows users to fill in a web form
and hit a send button. The order is built as a text attachment to an
email in a c# class, the email is then built (also in c#) and sent to a
specified inbox. 

Yesterday, some of the subject headers had [!] before the subject. Since
all the order emails are built and sent from the same program, I am
assuming that this must have been added at some point between
objMail.Send and arriving in the inbox.

Does anyone know what, how or why this might appear?

TIA
Les Lytollis