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Why do E-mails Take so Long to Deliver
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We have come to expect e-mails to take seconds to be delivered. We often regard e-mails as an instantaneous method of communication. Almost like a telephone call.

We are very surprised if an e-mail takes over one hour to deliver. But did you know it can take days? This article gives the reasons for the speed of delivery and also the reasons why delivery can take days.

Overview of E-Mail Delivery

Pressing the Send Button

After the e-mail has been written, composed and spell checked the send button is pressed. This causes the e-mail to be sent from the PC to a mail server known as a SMTP (Simple Mail Transfer Protocol) sever. This is usually operated by the ISP that you pay for your Internet connection or, if you are a business, the SMTP server than handles all outgoing mail. This rarely fails. If it fails it is usually because the SMTP server is down or the dial-up line is not connected.

Sender's SMTP Server Locates the Recipient's SMTP Server's IP Address

If the sender's domain name if different from the recipient's domain name the SMTP server cannot deliver the e-mail to the recipient. It needs to pass the message to an SMTP server that knows how to deliver the message. The sender's SMTP server tries to find the IP (Internet Protocol) address of the recipient's SMTP server. The Internet is a busy and congested place and this request could fail. The SMTP server does not give up at that point. It will try many times times before giving up. Most SMTP servers will try for five days before throwing in the towel! If you are lucky, some SMTP servers will inform the sender if the message cannot be sent within 24 hours, but still it will continue to try. Eventually, the SMTP will find the IP address of the recipient. Now it can try to pass the message on.

Sender's SMTP Server Passes Message to the Recipient's SMTP Server's

Now that the sender's SMTP Server knows the IP address (similar to a telephone number) of the recipient's SMTP server, it calls it and ask for permission to send the message. Most times this works but sometimes the recipient's SMTP server will reject the call. This can be for many reasons - such as the the server is too busy (especially when viruses are being propagated around the world). If it does fail, the sender's SMTP server will continue to try before giving up. Usually, the message will be sent to the recipient's SMTP server before the five days has passed. However, this is not the whole story.

A recipient usually has 2 SMTP servers. Some organisation have many more - AOL has at least 4. If the main server is busy, the sender's SMTP server will also call any other of the recipient's servers.

Recipient's SMTP Server Passes Message to another SMTP Server

If the message was not passed to the recipient's SMTP server than can deliver the mail, the message will be passed to another SMTP server that may be closer to the recipient. Like the previous section, this could take seconds, minutes, hours or even days. This situation occurs a lot in business where the main receiving SMTP server is owned by a third party and checks all incoming e-mails for viruses and spam. If they pass the tests, the message is passed to the recipient's SMTP server.

Recipient's SMTP Server Places Message into Recipient's Mailbox

At last, the message has arrived at a SMTP server that knows where the recipient's mailbox is located. Once the message is in the mailbox it can be seen and viewed by the recipient.

How Long Does it Take

In over 95% of cases the above process takes less than a minute. Far quicker than it took to read the explanation. In a few cases the message could take as long as 5 days to complete its trip from sender to recipient. It rarely takes move than 5 days as one of the SMTP servers will send the message back as undeliverable. And yes, the e-mail that contains the error message could take 5 days to get back!


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Last Updated: Monday, 04-Feb-2019
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