Why would email be delayed?

http://www.ehow.com/how_4614939_check-why-email-was-delayed.html

It is a common misconception that email delivery is instantly completed. Email is designed to be sent from Mail Server to Mail Server until it reaches the ultimate destination with no guarantee on how long it will reside at each intermediary stop. Most cases of delayed email are due to slow or overloaded mail servers. Email delivery delays are caused by spam, frequency of checking one's email, or other hardware issues. When an email is delayed for a significant amount of time, the best way to check why an email is delayed is to analyze the email header on the delivery delay notification to see where the delay is occurring.

It's also likely that it was intentionally delayed: http://www.byui.edu/help/outlookhelp/mailing/delayed.htm

I don't know about today, because I'm not in that loop anymore, but, when I was working for an ISP, I was very aware of the eMail parameters. It was not unusual for a normal eMail to be delayed by 15 minutes to a half hour because of the way that eMail is passed on the net. The primary reason for this is the individual ISP eMail parameters. I was working for a small ISP who was limited to sending eMail every 15 minutes. The timer that batched off the eMail was set to a minute that was not one of the primary quarter hours. Each ISP along the line is doing the same thing, most more often (something like every 10 minutes or 5 minutes depending on the size of the ISP). Because of this the automatic delay for an eMail could range from immediate to as much as an hour with no other reason involved.