[noduerme]

This is called "greylisting" and it's fairly normal if you don't run your own server. Someone triggers a spam block and gets put on a partial time-out. Then the more times they try to re-send an email to you, the longer their emails get kept in limbo.

If you don't control the server, you don't have independent email.

[capableweb]

Well, could be that, could be other things as well. Could be the sender who have implemented their email sending via a queue, and currently they are overloaded. Could be their email sending server/service who is behind and having delivery problems. Could be numerous things thanks to the nature of email.

In the end, the UX of having to wait for an email sucks, sometimes.

[noduerme]

True. The outbound queuing thing is going away, though. A lot of banks and service companies used to wait until a regular 2, 3 or 5 minute mark (00:00, 00:02) to send those verification emails in batch. In the past year or so there's been a move away from that it seems. The initial reason for batching them was to prevent spammers from triggering hordes of emails somehow off their system by gaming the "submit" button for a form, but now there's other security in place and the emails are being sent out faster. Generally, once an email is sent, it's almost instantaneously received if the recipient isn't delaying or blocking it. 99% of the time when I hear from clients who are waiting for an email, it either came in right away and bounced because our spam filters were too tight, or our own greylisting kicked in and slowed it down.