We are about to switch to a new forum software. Until then we have removed the registration on this forum.
I've found that Gmail was marking all Processing Forum mail notifications as Spam and I'd never realized it. Was I the only one? Looking at the raw mail I checked that there's no txt/spf data, and I've been dealing with that in my server.
Received-SPF: none (google.com: noreply@processing.org does not designate permitted sender hosts) client-ip=23.239.19.146;
Authentication-Results: mx.google.com;
spf=none (google.com: noreply@processing.org does not designate permitted sender hosts) smtp.mail=noreply@processing.org
You could try by adding v=spf1 a mx ~all
in TXT field of your DNS records as I guess your are sending mails from the same server, and see if it helps to not be marked as Spam.