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smtp - can use any other port other than 25?

Posted on 2011-02-16
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Last Modified: 2012-05-11
can smtp use any other port for outgoing mail? how does that get configured or checked?

thanks
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Question by:anushahanna
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Chewe Chileshe earned 668 total points
ID: 34908743
you can use any available port you like, as long as the sever recieving the connections is also configured to recieve smtp on the same port. Thats easy to do when using a relay server, however internet mail works on port 25 as a standard for smtp.

The configuration is dependent on what system you are using.
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by:anushahanna
ID: 34909393
how can we find what port is being used in relay server situation.

port 25 is for the exchange server or the client which uses the exchange server connection to send an email?
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by:jbordelon
jbordelon earned 1332 total points
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Port 25 is the open port that server listen for connection. The client should initiate tcp connection on port 25 and then negotiate a high port for return communication. In a Micosoft client like Outlook Express, you configure the SMTP port in the tools menu, accounts, properties, Advanced Options tab, and changing Outgoing Mail (SMTP) value.
You could change the server listen port by different methods depending the operating system, but you couldn't use ports that the server use for other purposes in the same IP address, by example if you use port 80 for SMTP, you cannot run a web server at this port with same IP. Microsoft IIS let you set the port that SMTP server use opening MMC, Internet Information Server, Properties, General Tab and pressing Advanced button. You could use more than one IP addresses in your server, one for SMTP at any port, and other IP for other services.
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by:anushahanna
ID: 34935831
>>then negotiate a high port for return communication.

why high port? 24 will work just fine, too, right? what is the reason for the 'high'
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by:jbordelon
jbordelon earned 1332 total points
ID: 34937810
TCP/IP protocol use source and destination ports. http://en.wikipedia.org/wiki/Transmission_Control_Protocol
When a connection is stablished, two ports are used. One of this ports is defaulted to a service (Well-known ports: 0–1023 http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers ), for example 80 to http, 25 smtp, 21 ftp, 53 dns, etc. Otherwise you wouldn't know at wich port initiate a connection for a protocol or service. The return port could be any unused port in the range 1024-65535 and is called Ephemeral port: http://en.wikipedia.org/wiki/Ephemeral_ports
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by:anushahanna
ID: 34985854
thanks.
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