>I was initially thinking that getting a success/fail result from sending an e-mail would be nearly instantaneous. However, now that I think about it I once set up a mail server myself whose polling interval is 1 minute, so getting a response from that one could take at least a minute. So yes, the problem of pending updates is real.

It is far more than that. Email servers would in average return a bounced email in four days. So, that could be up to four days, in most cases, when a reject applies. And, that is valid as long as the email server would send a bounce email where some of them, in some cases, won't send anything.

>I think we may end up requiring registration for those (relatively few) users who will want to post comments. If they don't want to comment, they don't have to register.

Yes, that would be the way to go. If the registration process is done in an easy way, such as step 1, 2, 3, it would make the process of the registration much more pleasant and could work ok for you in that case.