has it been this message:
Dec 27 14:59:12 server postfix/smtp: 41A0891378B: to=<email@example.com>, relay=uswgco33.uswest.com[18.104.22.168], delay=1636, status=bounced (host uswgco33.uswest.com[22.214.171.124] said: 553 5.1.8 <firstname.lastname@example.org>... Domain of sender address email@example.com does not exist (in reply to RCPT TO command))
Dec 27 14:59:12 server postfix/cleanup: A769891378A: message-id=<20051227205912.A769891378A@server.roadracers.o rg>
there seems to be a problem with the domain of your server, maybe the dns-settings were recently changed. perhaps the mailserver where your email went to could not look up the domain and therefore didn't accept the mail ?
but I am only a little noob, falko or till might know more about this case