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I had this idea from http://stackoverflow.com/questions/2813326/maximum-number-of-bytes-that-can-be-sent-on-a-tcp-connection

is it possible that smtp servers like gmail and yahoo enter into some form of agreement to maintain a tcp connection between them so that lots of mails could be sent on the same tcp connection.

it would be efficient as there would be heavy mail traffic between these mail servers.

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3 Answers 3

As the overheads of establishing a connection are absolutely trivial compared to the data that is to be transferred there is no real benefit in going outside conventional protocols and maintaining open connections. Neither Yahoo not Google are a single system. Each of their servers is an independent unit and will transfer mail in exactly the same way as any other mail system. Don't let their overall size cloud your view of the basics.

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your point seems valid. i'll wait for others comments. –  Rohit Banga May 12 '10 at 5:20
    
there would be an smtp handshake also for each mail that is exchanged. smtp allows mail from and rcpt to requests to initiate mail transfer. so that does not violate the protocol. only thing is that they should not terminate a long running tcp connection. –  Rohit Banga May 12 '10 at 5:22
    
The handshake is only required per batch, not per message. That way large numbers of messages for the same destination can indeed be transferred quite efficiently. There are good reasons not to allow a remote system to maintain a permanent connection. e.g. DOS prevention. –  John Gardeniers May 12 '10 at 5:54
    
batch? that is what i meant by permanent tcp connection. sending lots of mail in a single tcp connection. –  Rohit Banga May 12 '10 at 8:04
    
As a typical batch will take less than a second or two I really don't think that qualifies as a permanent connection. –  John Gardeniers May 12 '10 at 8:55

I'm sure mail servers (Exim in my experience) can be tuned to pass multiple messages on the one connection.

This is obviously only relevant if the destination MX is the same for all messages.

http://www.exim.org/exim-html-4.10/doc/html/spec_29.html

^^ That page talks about the configuration options for exim which effect this.

T

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I've never heard of providers doing this, though it is very likely that these providers peer directly with each other to exchange mail, thereby avoiding bandwidth transit costs.

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Very likely? For SMTP? ... –  Warner May 12 '10 at 4:54
    
Highly unlikely, or necessary. They each run many systems, each of which transfer mail separately and independently. –  John Gardeniers May 12 '10 at 4:56

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