Can't activate if mail server uses greylisting
If you have a mail server that supports grey listing (rejecting 1st appearance emails with 450 code which means temporary failure i.e. disk full), then you can't join enworld.
The site doesn't drop the message in a sendmail or postfix queue, it appears to use outbound mail MTA that doesn't save failed messages for later.
This means the message comes into the mail server and the server rejects it:
451 4.7.1 Please try again later. Session Greylisted
Since all messages starting with 4xx mean "hey I can't take this now, but I can later" all mail servers will retain the message for 4 days and keep trying every 15 minutes.
I spent a week looking for a way to resend the activation. I had given up, then I noticed a 6 point font in the middle saying I was a registered user and click her to resend the activation codes.
These codes need to be prominent and/or inside the control panel, since I never saw it until after this post and I looked for days inside the control panel (or on the page saying you need to register to see this or that.)