I just found that amavisd-new is quarantining almost all the activation and registration emails sent from wpmu usong PHPMailer with the following reason:
X-Amavis-Alert: BAD HEADER SECTION Duplicate header field: "MIME-Version"
and indeed the mail contains:
X-Priority: 3
X-Mailer: PHPMailer (phpmailer.sourceforge.net) [version 2.0.2]
MIME-Version: 1.0
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="UTF-8"
To activate your user, please click the following link:
so the MIME version is declared twice. How to fix this? is it a Phpmailer bug or rather something that needs to be fixed within wpmu?