Honestly, when registering for sites, whether it be for phpBB or anything else, there are plenty of times when I do not receive the registration email. I'm not talking about it arriving hours later or it ending up in the spam folder. Rather, I'm talking about emails quite simply not showing up or in some cases, showing up with the activation code missing.
It seems to me that automated emails fail a significant amount of the time for whatever reason, especially when sent to a Gmail or Outlook address.