Emails from custom domain going to spam

Solution 1:

There are a few ways to approach this, none with 100% certainty, but if you try several you'll at least improve your delivery rate. I'm going to assume you have control of DNS for your domain. This is not an exhaustive list.

• Have an MX record for your domain, so you're able to accept responses.
• Set up an SPF record for your domain. This isn't too hard if you know the sources of all email (e.g. you'll always be sending through the same subset of SMTP servers). Microsoft encourages submitting your record to them after setting up SPF, which helps you get through to Hotmail.
• Check to see if your sending IP address is on any blacklists. Get it off if you can; this process is different for nearly all of them, though.
• If you can, set up DKIM authentication.

The last part is just to send "good-looking" email, and this is going to vary a lot depending on how you're creating your messages (in a webapp or in your own desktop client). Docunext's answer is an example of how to approach that problem. This includes things like having the reply-to and from headers match, not using blank subject lines, etc.

Solution 2:

You'll have to find out why they are going into their spam folders. Depending on what filtering system they use, the scoring might get added to the email header. Ask they forward you the email as an attachment, then view the original.

If you are lucky, you'll see something like this:

X-Spam-Flag: YES
X-Spam-Checker-Version: SpamAssassin 3.2.5 (2008-06-10) on
    pro-12-gl.savonix.com
X-Spam-Level: ******
X-Spam-Status: Yes, score=6.2 required=4.0 tests=BAYES_99,HTML_MESSAGE,
    MIME_HTML_ONLY shortcircuit=no autolearn=disabled version=3.2.5

In this case, the BAYES_99,HTML_MESSAGE,MIME_HTML_ONLY spamassassin rules caused it to get classified as spam. The other answer mentioning rdns also has a rule, but many ISPs simply block emails sent from an IP without an rdns, or from an IP with an rdns that doesn't match the name of the SMTP server.

Solution 3:

You didn't mention your email service provider.

Did you register a fresh domain name or an old one mate? If it is an old one, do a Google search or use Internet Archive to see whether your domain is blacklisted or affected by any Google algorithms or any such hit lists.

If your domain's previous life was a spammy one then probably it could affect your emails as well. Also setting up an SPF record for your domain name will increase trust.