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I'm doing a bit of research for work regarding SMTP servers. Basically, our company is bringing some work in-house that we've been doing remotely through other companies. We have the need to work with mailing lists, but I'm not sure how exactly to handle the great number of emails that will be sent out. From what I'm reading, many applications boast an "internal smtp, so you don't use your ISPs," but that just confuses me.

What exactly is an SMTP server, and how does it bypass my ISP? I can't seen to fathom how anything could make it from my network, to the recipients without taxing my ISP's servers. Can somebody shed some light on this?

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up vote 3 down vote accepted

What they mean is that it doesn't use your ISP's SMTP server as a mail relay. The so called "internal SMTP" server can talk directly to the recipient's mail server to deliver the message.

Simply put, an SMTP server is an application that knows how to communicate using the SMTP protocol, accepting messages from clients and other servers and routing them on their behalf.

Your SMTP clients will need a server to connect to in order to deliver the mail they are sending out. This where an SMTP server comes in. They (the clients) can be configured to connect to your ISP's server, or they can connect to an internal server, or any other that is configured to allow delivery for the specified domain.

The SMTP server will be responsible for doing the dns resolution that is required for discovering which mail (SMTP) servers are responsible for accepting mail for your recipient's domain and routing it along. By using your ISP's you let them manage it. By bringing it internal you are accepting the responsibility of managing the server, but you also get the added benefit of being able to control the configuration.

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An SMTP server is just a server that handles the transport of mail. The server you configure in your email client as the "outbound mail server" is an SMTP server. So is the server that receives the mail and delivers it to the recipient's mailbox.

They just mean "bypassing your ISP's SMTP server". If you run your own mail server, it can send mail directly to the recipients mail server without traversing your ISP's server. This is good if you're sending bulk as most ISP's would get rather perturbed if you were to suddenly dump 100K outbound messages on their servers.

However, note that you do not have to take the server in house to solve that problem. There are many companies that specialize in high-volume mail that would be happy to handle all of your outbound SMTP needs.

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Is there any reason not to do it in-house with 100k+ emails? – Sampson Sep 3 '09 at 20:04
With your own SMTP server, you have to worry about deliverability: keeping your IP off of blacklists, not pissing off customers at big ISPs, dealing with complaints, etc. An outsourced company will do all of that for you. – Bill Weiss Sep 3 '09 at 20:06
Yes. Your IP and or mail domain could be blacklisted if too many people flag the messages as spam. Also, if you're sending from and there is no SPF record in DNS, the mail may be rejected. If it's mass mailing you want, it's probably best to leave it to a professional company that specializes in such. – Marcus Sep 3 '09 at 20:07
@Bill: Thanks. I definitely don't need more on my plate. – Sampson Sep 3 '09 at 20:09
@Marcus, et al., Any company in particular you could recommend? – Sampson Sep 3 '09 at 20:10

An SMTP server is a server that is setup to either receive or send (or both) emails. You can setup your SMTP server to communicate directly with remote servers to send email or you can use your ISP's SMTP server as a smart host. In this scencario, your SMTP server sends messages to your ISP's SMTP server and your ISP actually sends them to the remote server. This action is called relaying since you are using a third party (your ISP) to send mail to a recipient on your behalf. The message headers on an SMTP message will show all the SMTP servers it passed through to reach its destination.

When they say "bypassing" your ISP, they don't mean the traffic won't touch the ISP. They simply mean you aren't having their mail servers send stuff for you.

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