Well, we'll need vastly more information in order to give you any shread of help.
Please start with the version of Messaging server. Run
imsimta version
and post that result.
then:
1. What's an "E-mail announcement"? How are these generated?
2. "with HTML tags". Are you saying that the message has a mime part stating it's html?
3. Can you post the entire contents of one such?