This is another side-effect of the hosting company changing owners and migrating to different servers.

I suspect this has been happening for a long time, but a user has only recently contacted us letting us know they got an error message when attempting to join.
 
I've contacted the hosting company to fix it, and I'll update here when it's working.
 
Note:
There were a LOT of "pending verification" users, so I suspect a lot of people have had email issues over the years. I've deleted all these failed new users, so the email addresses and usernames should no longer be "taken" if those same people try again (but just wait until I've posted here that emails are working again ).

Ok, after a whole lot of messing around, it looks like the error people are seeing is because our SMTP mail server is rejecting their email address for some reason.
 
Some possibilities are:

• the recipient's email address was typed wrong or doesn't exist
  
• the recipient's mailbox is full
  
• the recipient's mail server is rejecting mail from this site because the sender (this site) isn't using authentication
  

That last one is tricky. I just can't seem to get the site to send email using authentication, and the mail server settings provided by the host don't seem to work. 

I'll add a little note to the Join page, but there's not much else I can do until I can reliably get it to fail with a known good email address.
 
~ Andrew