un-block messages w/ blank subject line

Windows 2000, Exchange 2000, Symantec AntiVirus Enterprise 8.5 -fine.

2 weeks ago added filters to quarentine attachments with certain extensions, and add subject tag on incoming mail with possible spam/viruses. Also set black list to use spamhaus.org. – fine I thought.

But, incoming or outgoing mail with blank subject lines are being blocked. no warning, no message to sender, no admin message. I turned off all the filters I turned on – but blank subject line messages are still being blocked.

Any help would be appreciated.