The reason this is occuring is because some spam filtering software is screening E-Mail for the "X-library:" header.  This header is sent by Indy 8.0 and the release versions of Indy 9.0.  In and of itself, this is not bad.  However, some malware such as spam bulk E-Mails and even a worm (W32/Nicehello@MM), were built with Indy meaning that they sent the same "X-Library:" header that most Indy-based E-Mail programs do.

For this reason, we have decided to remove the X-Library header from Indy 9.0 code and you can obtain it from our VCS.  Instructions are at:

http://www.indyproject.org/download/DevSnapshot.html

If you are using Indy 8.0 and you can not upgrade to this code, you can remove the X-Library header by editing IdMessage and removing line 439 which reads:

Values['X-Library'] := gsIdProductName + ' ' + gsIdVersion ; {do not localize}

and then rebuild Indy using the FULL*.BAT file for your system.