There are three types of domains that are used when sending an email.
1. Email Domains
By email domains, we mean the domain that the email physically is sent from.
When we send emails on your behalf we will ordinarily use one or more of our own generic domains to physically send your email from.
We do this because mail servers expect emails to come from domains that have had specific DNS records set up telling them that it's okay for the emails to be sent from this mail server.
If these DNS records are missing, nine times out of ten the email will be rejected.
You may well have received marketing emails yourself in the past and noticed that the FROM address might say something like From email@example.com on behalf of ourmailsender.com. This is exactly what appears when the email has been sent from an ESP and the user has not authenticated their domain.
Not only does this look unprofessional from the end-users' point of view, but it also means you are sharing domains with lots of other people, and this is not ideal as if another customer causes one of our domains to be temporarily blocked your own campaigns may be affected.
Authenticating your own domain means emails are sent from your domain, preventing email clients from showing that nasty "On behalf of" message and allowing you to remain independent from any abuse that our platform as a whole may suffer (rarely).
2. Tracking Domains
When you click a link in a campaign email a subscriber is quickly forwarded to one of our servers which then tracks the link and then forwards them to the final website.
By default, we use one of our own domains to track links. As with the email domains above, if another user has caused one of these domains to become blacklisted then your own campaigns can be affected.
This is rare, but it does happen so adding your own tracking domain is a great way to ensure maximum deliverability and inboxing.
3. CDN Domains
If you design emails and add images by uploading to our servers when a customer receives this email they will be downloading the image from our server and this is done by using a CDN domain.
As with tracking and email domains above, we use one of our own domains which are pointed at our image hosting servers in each email you send. However, using a shared domain can cause issues if another of our customers has caused a blacklist with the domain name.
This is rare but if it does happen it can cause issues with your own campaigns reaching the inbox so the best way to ensure your emails always get delivered is to point your own domain at our image hosting (CDN) servers.