From Firetrust Internet security: -

"any email that you open with MW is opened in text only mode, and cannot transmit any data, run any program or anything like that, so MSGTAG cannot work in MWpro, however if you download the email to your email client, it'll work like normal.

Benign, (another of our programs) strips out any code from the email however, so you can download the email to your email program and MSGTAG wont work in the email.

Details here:

http://www.firetrust.com/products/benign/support/frequently-asked-questions#how-do-i-block-msgtag-read-receipts"