For about a week now, forum notifications come from a new email address: noreply@aqua-mail.com.
Previously the forum was configured with my personal Gmail account. This was causing a warning in Gmail / web ("forged headers" or something like it).
I've also enabled SPF and DKIM, which are technologies that confirm (via DNS entries and digital signatures) that these messages really do come from where they claim they do, and that the mail server is authorized to send them.
These changes make Gmail and other mail services much happier, and significantly reduce the likelihood of forum notifications being mistaken for spam.
