Let's just say that CathInfo is hosted on a "small business server", and some high-brow servers consider small-scale web servers like mine to be a SPAM risk.
So some domains (sbcglobal.net, aol.com, mail.com, and others) reject ANY and ALL e-mails originating from my web server -- that includes any websites hosted on my server.
I spent lots of time looking into how to fix it, but I ran into a brick wall. My Internet Provider would have to cooperate in a certain way, but they refused to do so unless I upgrade to "business Internet". The long and short of it: I'd have to spend $250 a month on Internet instead of $80 a month. So that's a no-go. I'll just have to live with some members not getting any activation e-mails, thread update e-mails, password reset e-mails, etc. from CathInfo.
I have activated about 12 new user accounts that have accumulated over the past several weeks. I'll try to remember to do this more often in the future.