Q. Can anyone tell me why it would take several days for a member to receive an email informing him that there was a reply to his posting?
A. The email notifications may be delayed by his ISP because mail sent by a script is not considered "High Priority" so they will queue mail sent by scripts to prevent them from clogging the system from higher priority mail.
They are individual servers amongst THOUSANDS in a datacenter. When one server sends out an email, it still spits it out to the INTERNET saying "send this message to so and so" the sending server doesn’t know the "Recieving" server is sitting 3 cubicles down.
For example.... an email is sent out from americanglassgulid.com to your site firstname.lastname@example.org.
americanglassguild.com is on "Server A"... and email@example.com on "Server B"
Server A sends the email saying "I want to email firstname.lastname@example.org, please deliver". It doesn’t know that your site is on server B, in fact it doesn't know there is even an email@example.com AT ALL. All it does is send the data out saying "Please deliver this to XYZ"
The email goes out on the net and finds a directory server and the email says to the directory server "I am looking for firstname.lastname@example.org, where do I go?" . The directory server says "Oh they are over here, make a left on Oak Street, a right on Maple and they are in the red house... look for "Server B".
So the email passes through a couple others and arrives at Server B and says "I am looking for email@example.com” and the server says "3rd floor".
Conceptually, you would think because the servers are in the same building they would just pass data back and forth freely, but that is not how it works. One server does not know what is on another server. A server just sends out data and receives data that is sent to it. It has no idea what or where any other server is. That is what DNS servers are for; they are the road maps to the internet.
Another way to look at it is. If you wanted to send a package to your neighbor via the postal service. You give the mail to your postman, it still has to go through the "central office" to be processed and delivered. He doesn't just walk next-door and give it to him right away.