If we're talking about a five minute delay in email, the user may get bored of waiting and decide not to log in. If we're talking an hour long delay, the link i

If we're talking about a five minute delay in email, the user may get bored of waiting and decide not to log in. If we're talking an hour long delay, the link in the email may be expired and won't work any longer. I think I've seen email links that expire as quickly as 15 minutes. It's possible some mail servers would have very short retry windows, so if your mail server was offline briefly a sending server could give up trying and fail the delivery. For a login link that expires quickly, it's probably not worth retrying for several hours or days.