Any number of a thousand things can cause email delays. I'll throw one possibility out there - I work with an ISP who uses a spam filtering service. The mass volume of spam being thrown at all the various users causes emails to get hung up in the mail queue while it gets processed. Obviously this can be overcome by multiple servers and faster machines, but corporate has to weigh the facts: the cost of such upgrades against what's being earned on all those mail accounts. Sometimes email just doesn't provide enough income to support it.

That's true. Even though I didn't think it was happening with one ISP I'm with, I have seen massive delays before.

On a different note, I was just informed by one ISP that in an effort to alleviate bottlenecks and other delays, they are now limiting the number of emails sent to no more than 25 every 30 min; and not more than 300 per day.

Guess I have to find another way to send out my newsletter to my thousands of subscribers.

There are a lot of reasons but here are a couple that I have seen in the past:

Do you have reverse DNS set up? If you don't, this can cause some problems. If the ISP makes the client use their SMTP server and not yours, this can also cause some delays

-Corey