starting several weeks ago, GMail blocked any mails originating from our board:
Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been blocked. Please visit http://www.google.com/mail/help/bulk_mail.html to review our Bulk Email Senders Guidelines
Means, email notifications of PMs or forum updates won't find their way to GMail users anymore.
Sorry for related inconveniences.