Iis – How to configure IIS[SMTP] & DNS of our website so that the emails are not marked as spam

Be sure that your emails don’t look like typical spam emails: don’t insert only a large image; check that the character-set is set correctly; don’t insert “IP-address only” links. Write your communication as you would write a normal email. Make it really easy to unsubscribe or opt-out. Otherwise, your users will unsubscribe by pressing the “spam” button, and that will affect your reputation.

On the technical side: if you can choose your SMTP server, be sure it is a “clean” SMTP server. IP addresses of spamming SMTP servers are often blacklisted by other providers. If you don’t know your SMTP servers in advance, it’s a good practice to provide configuration options in your application for controlling batch sizes and delay between batches. Some mail servers don’t accept large sending batches or continuous activity.

Use email authentication methods, such as SPF, and DKIM to prove that your emails and your domain name belong together. The nice side-effect is you help in preventing that your email domain is spoofed. Also check your reverse DNS to make sure the IP address of your mail server points to the domain name that you use for sending mail.

Make sure that the reply-to address of your emails are a valid, existing addresses. Use the full, real name of the addressee in the To field, not just the email-address (e.g. "John Doe" <john.doe@example.com> ) and monitor your abuse accounts, such as abuse@example.com and postmaster@example.com.

You need to make sure that your email server is setup properly so that these mails are not marked as spam. As mentioned, one of the most common reasons is Reverse DNS. Most of the big providers require that you have a correct RDNS pointer record setup for your mail server before they will receive mail for you.

You also want to check that the IP your provider has given you has not been blacklisted, use a facility like this one to check. If it is on the list, if it's a new IP you have been given you can probably get your provider to give you a new one, if you've had it a while then it will be harder to prove that you are not the ones who got it blacklisted.