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.