Our system firewall+scanner has been blocking mails for new visitors & forgotten passwords for the past 5 months. Issue wasn't easy to notice, since not all were blocked - but at least 30%. The issue is now fixed. I wish I'd noticed it earlier.
Also sent an automated Welcome -email to all new 1000+ members with a possibility to login (and reset a password). We'll see a rush of neewbies from nowon, so please welcome them here by being helpful!
What I'll now try to fix is also find out who has ordered password reset and re-sent them an email as well.