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
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.
It may be obvious to some but always check to be sure your antivirus/antimalware/antiexploit software isn't causing issues. I had a local smtp server configured to deliver through an office 365 account and malwarebytes was blocking any processing from happening in the mail queue folder. Once I whitelisted the inetpub directory my smtp server started working again.