Avast 5.0.462 Outgoing mail reject message?

I received the following message after sending mail and mail not sent. After I turned off mail shield the message was sent properly.

Your message did not reach some or all of the intended recipients.

  Subject:	Newsletter info from the Friends
  Sent:	3/23/2010 2:22 PM

The following recipient(s) cannot be reached:

  'townadmin@wallingfordvt.com' on 3/23/2010 2:22 PM
        554 Suspicious message

I sent a copy to myself and when receiving I receive a heuristics warning

Suspicious whitespace sequence

My final conclusion is both issues (sending & receiving) were caused by the name of the file that was attached. I changed the name of the attached file and no more errors sending or receiving. I had left the default name Microsoft Word had given it which typically is part of the text in the first line which was Hart Lines April 2010. The spaces did it in.

???