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is emails through internet guaranteed???

Posted on 2000-02-21
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hi guys,

I was having discussion with my friend that emails is or is not guaranteed through the net. I was on the side that said NO as I said that is NO email protocol in the world that can guarantee 100% delivery since the internet is SO large and that the packets could be easily lost. Am I right or wrong? Please explain....

Thanks
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Question by:Haho2
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klover earned 300 total points
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Nothing is 100%, but email is more reliable than the post office.  (Not to mention faster and cheaper!!!)  Email could be lost at any point where delivery was confirmed, and then there was a failure before the mail could be relayed.

When email leaves your computer it is sent to your Internet provider's relay mail server, called an SMTP server.  If this server is not available you will be notified immediately.  The mail sits in the outbound queue of the SMTP server while the server queries a centralized database of addresses (called DNS, or Domain Name Servers) to find the destination.  In other words, haho2@aol.com, would be resolved to a real IP address via DNS.  Here are some sample DNS entries from AOL...

aol.com      MX preference = 15, mail exchanger = zb.mx.aol.com
aol.com      MX preference = 15, mail exchanger = zc.mx.aol.com
aol.com      MX preference = 15, mail exchanger = zd.mx.aol.com
aol.com      MX preference = 15, mail exchanger = yb.mx.aol.com
aol.com      MX preference = 15, mail exchanger = yc.mx.aol.com
aol.com      MX preference = 15, mail exchanger = yd.mx.aol.com
aol.com      MX preference = 15, mail exchanger = ye.mx.aol.com
aol.com      MX preference = 15, mail exchanger = yg.mx.aol.com
aol.com      MX preference = 15, mail exchanger = yh.mx.aol.com
aol.com      internet address = 205.188.146.23
aol.com      MX preference = 15, mail exchanger = za.mx.aol.com
aol.com      internet address = 205.188.160.121
aol.com      nameserver = dns-01.ns.aol.com
aol.com      nameserver = dns-02.ns.aol.com
dns-01.ns.aol.com      internet address = 152.163.159.232
dns-02.ns.aol.com      internet address = 205.188.157.232
zb.mx.aol.com      internet address = 152.163.224.35
zb.mx.aol.com      internet address = 152.163.224.36
zb.mx.aol.com      internet address = 152.163.224.37
zb.mx.aol.com      internet address = 152.163.224.33
zb.mx.aol.com      internet address = 152.163.224.34

Notice that there are several AOL mail servers for your provider's relay server to choose from.  It will try connecting to the IP address of the first one on the list with the lowest preference, then try subsequent servers if the first one is unavailable.  After several attempts, (sometimes over the course of 24 hours) if none of the servers are available, you'd get a NDR, a Non Delivery Reciept via email from your ISPs mail server. However, if there were a crash or glitch with your ISPs relay mail server during this step, your mail could be lost.

Your mail could also be lost if there was a glitch with AOL's mail server before the mail was retrieved by your intended recipient.

When sending an email, you can request "Delivery" and "Read" receipts.  The method of doing this varies depending on the program you are using.  When you request a delivery receipt, your ISP's relay mail server will send you an email stating that your mail was delivered to AOL's mail server.  When you request a read receipt, AOL's server will send you an email when your message is downloaded by your recipient.

In summary, there can be all kinds of failures in the system, but there are failsafes and notification features in place so you know if there was a problem, and where it occured nearly 100% of the time.
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