Empty message body

Over the past couple of weeks I've noticed that some incoming messages
have an empty body. These are messages from user groups and it seems very
random. Yesterday I tried to create a YouTube account and part of that
process is for me to click on a link in an email message sent to me from
YouTube. The first email I received had an empty body, so I tried again 4
or 5 times. Every one of these messages has an empty body. I use the
latest Sendmail with UWimap, Spamassassin and Clamav on linux with
Procmail and Thunderbird on my local machine, also linux. No other mail
tools are running. My maillog shows the following...

Jan 29 12:56:19 mail sm-mta[11361]: m0TKuINj011361:
from=, size=5965, class=0, nrcpts=1,
msgid=<20080129204316.903F85AC60@youtube.com>, proto=ESMTP, daemon=MTA,
relay=sjl-smtp12.sjl.youtube.com [208.65.153.16]
Jan 29 12:56:24 mail sm-mta[11361]: m0TKuINj011361: Milter add: header: X-
Spam-Status: No, score=0.0 required=3.6 tests=DKIM_SIGNED,DKIM_VERIFIED,\n
\tHTML_MESSAGE,SPF_PASS,WHOIS_NETSOLPR autolearn=failed version=3.2.3
Jan 29 12:56:24 mail sm-mta[11361]: m0TKuINj011361: Milter add: header: X-
Spam-Checker-Version: SpamAssassin 3.2.3 (2007-08-08) on mail.rlknight.com
Jan 29 12:56:24 mail sm-mta[11361]: m0TKuINj011361: Milter add: header: X-
Virus-Scanned: ClamAV 0.91.2/5599/Tue Jan 29 08:27:10 2008 on
mail.rlknight.com
Jan 29 12:56:24 mail sm-mta[11361]: m0TKuINj011361: Milter add: header: X-
Virus-Status: Clean

Can someone tell me what's causing this and how to fix it?

Thanks,
Rick