Why is my e-mail sometimes slow to arrive?
Sometimes incoming e-mail from new senders can be delayed; the reason is because we operate a system called greylisting on all our mail servers which has proved extremely effective at combatting spam with almost no false positives.
It works on the principle that, for the most part, spammers simply "fire and forget" their messages and if the first attempt to deliver a message is unsuccessful they just move on, most of their delivery address are bogus or obsolete so they can't waste resources on queueing mail for subsequent delivery. Conversely, rules compliant mail servers will try again a number of times before giving up completely.
So it's quite simple: if a message arrives here that is from a new sender, or to a new recipient, we return a message to the other mail server that says "sorry we're busy, please try again in a few minutes". We can't control how long it is before the sender's mail server chooses to try and send the message again, but provided a a few minutes have passed we will let the message through the second time. We then store the details of the sender and recipient for over a month so further messages from them pass through immediately. The spammers generally don't retry and so their unwelcome mail never enters our system.
In practice you will see no discernible delay as most of your mail arrives from regular senders, and even with mail from new senders we find most mail servers try again quite quickly - typically in under 10-15 minutes. Probably the only time you will notice a delay is if you are waiting for something like a password reset e-mail, and in this case the best way to hurry it's arrival is to issue a second request for the password reset after 5-10 minutes which will then enter our mail system right away. You can then ignore the original requested mail when it shows up later.
We highly recommend continuing the use of greylisting, but as of the 9.5 control panel version (September 2010) you can opt out at Mail Accounts > Mail Settings