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We contracted with a 3rd party vendor to build and host a web application for us. The web app will need to send email notifications out to users when they request it. The application is not hosted on our network, but it is reachable via a sub-domain we set up.

What options exist for sending the email? I know that we could have the application connect to the SMTP servers on our network and send the messages from an account setup there.

Is it possible to send the messages from the externally hosted server and not have them get eaten by every spam filter on the net? Would setting up SPF, Domainkeys, and/or DKIM address this? And if so, would it be just a good as sending the messages through the SMTP servers on our network?

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Personally I would configure the application to send email through your email server using authentication. That way you've got one ingress/egress for your email, you've got fewer services on the web server to setup and configure, you've got simpler troubleshooting to do when emails fail to be delivered, and you've got less to deal with in terms of your email being potentially marked as spam.

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Yes, it should be possible (assuming the 3rd party cooperates) to install and run an SMTP server on your web server. IIS includes an SMTP server that can be configured and used.

Regarding spam filtering, there is no guarantee as all spam filters behave differently and have different rules, criteria etc. But here's what I would recommend:

Set up a sub-domain like:, and create an MX record that points to your web server, as well as PTR record (these are both important components that spam filters often consider). Then make your web app send email from someting like: Also, set up a SPF record for the sub-domain which (obviously) includes the web server.

I would also configure the SMTP on IIS to reject all incoming email.

Like I said, this won't guarantee that you don't get dinged as spam, but it gets you a pretty decent chance to get past the filters.

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Since the webapp is built and hosted by a 3rd party vendor, you could use a 3rd party vendor for email too.

Sendgrid for example can be setup to send emails from your webapp, on your behalf, and you don't need to worry about spam filters etc. They take care of deliverability + you have access to very useful analytics.

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