Now that email has become so prevalent, most of use have come to expect email to be almost like a telephone call - instantaneous!
But did you know that emails can occasionally take days to deliver? How come?
Once you click on your email clients send button, the process begins. The mail message is sent from your PC to an SMTP mail server (Simple Mail Transfer Protocol) This server is usually provided by your ISP (Information Service Provider) - that is, the business that provides your internet connection. Some larger firms have their own SMTP servers. This initial connection is usually reliable - failure is normally only a problem in the case of hardware failure or lack of connection.
The SMTP server attempts to look up the address of your intended recipients own SMTP server. If your server and the intended recipients server are not one and the same, your SMTP server cannot itself deliver your message. Instead, it must relay the message on to another server that knows how to deliver it. So your server trys to contact your recipients server. This request could , of course, fail, since the servers may be thousands of miles apart and the route between them complex.
However, if it does fail, your own SMTP server doesnt just give in. It will continue to try over and over again to establish the connection - most will attempt this for five days before giving up. Often, your mail server will periodically report that its having difficulties, just to keep you informed! Hopefully, though, your server will establish the connection without too much difficulty.
Once the connection is made, your server will ask for permission to send the message. Usually this is granted, but occassionally the request is rejected. Sometimes, the recipients server is too busy at that time, but there can be many reasons. Once again, your server will keep trying before giving up.
Quite often, a recipient has 2 or more SMTP servers. If your message cant be successfully delivered by the recipients main server, the message may be passed to one of the others - and again, there can be further delays, especially if servers are checking incoming mail for viruses and spam.
At last, the message has arrived at a SMTP server that knows where the recipients mailbox is located. Once the message is in the mailbox it can be seen and viewed by the recipient.
Most of the time, this whole process is completed in under a minute. In a few cases, the message could take as long as five days. It rarely takes more than 5 days as one of the SMTP servers will send the message back as undeliverable. And error messages can also take 5 days to deliver!
