Email domain best practices

Domains in emails are important for identification, trust and reputation. But emails can embed many domains used for different purposes. They are listed below:

  • IP hostname
  • Envelope sender domain
  • From header domain
  • Reply-To header domain
  • Sender header domain
  • Message-ID domain
  • DKIM signing domain
  • Image URI domain(s)
  • Link URI domain(s)

This post provides some background on these email domains and gives considerations for choosing the right domains for your email.

IP hostname

The IP hostname is often assigned independent of the domains used in the email. A common scheme is to use a subdomain of the organization domain. For example mta01.company.com, mta02.company.com, etc.

Envelope sender domain

The envelope sender is communicated in the SMTP MAIL FROM command. It is the address where bounces are sent to. The receiver preserves this address in the Return-Path header when the email leaves the SMTP domain and is delivered in a mailbox.

For ESPs, it is common to use a common domain owned by the ESP in the envelope sender for all customers. For private systems it is better to use a domain that is “aligned” with the From header domain. This is a requirement for DMARC compliancy.

From header domain

The From header domain is the most important domain in an email, since it the only domain that is immediately visible to the recipient. The From header domain should be directly related to the identity responsible for the content and the sending of the email. In the context of DKIM it is also called the “author domain”.

Because the visibility of the From header it is important for recipients to recognize and trust the domain. Don’t do what phishers do, using variations of your main corporate domain. Instead use a subdomain and prevent abuse of your domain with DMARC.

Reply-To header domain

The Reply-To header can be used to receive manual replies on a specific address. Automated replies, e.g. out-of-offices are most often sent to the From address.

The Reply-To header domain is not relevant for authentication, so you can choose any domain. However the Reply-To header domain is often the same as the From header domain.

Sender header domain

The Sender header is used to indicate that the email is delivered from a different domain than the From header domain. The header can be used for Sender ID compliance, but it will result in a “Sent on behalf of” being displayed in certain email clients.

The Sender header is mostly used by low-entry email services and web forms for tell-a-friend or viral marketing. Now that Sender ID is replaced with SPF and DKIM this header does not have much use anymore.

Message-ID header domain

The Message-ID header contains a unique message identifier. It is recommended for the identifier to contain a fully qualified domain name. Some providers seem to take notice of that. Typically the domain part of the identifier matches the hostname that generated the emails or that sent the emails.

DKIM signing domain

The DKIM signing domain (d=) is not required to match any other domain in the email according to the DKIM standard. Many ESPs sign with a common domain owned by the ESP, so they can get into the FBL easily. This is called a “third-party signature”.

In order to be DMARC compliant the signing domain must be aligned to the From header domain. Aligned means that both domains share a common organizational domain. An “author signature” is a DKIM signature where the signing domain exactly matches the From domain.

Hotmail only validates author signatures. So this is also a good reason to make sure that the DKIM signing domain matches the From header domain. They also seem to validate signatures that match the Sender header domain, but only under certain circumstances.

Image URI domain

The HTML content of the emails will contain URIs in image tags. The domain used in these URIs can be anything in theory. Many emails use various domains as images are pulled from different sources. It is recommended not to use too many external image sources, as this is also a common characteristic of spam or phishing mails.

Link URI domain

The email content will also typically contain a number of links to landing pages. The target links are often hidden behind a service that registers the clicks and redirects to the target URI. It is recommended not to use link shortening services, as spammers commonly use these for hiding the target links.

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