TRACKING DOMAINS: If you have mailwizz installed on email.a.com and you send campaigns, all your url links will start with http://email.a.com and this is fine for most of the cases, but what if you want to handle emailing for b.com domain? In this case, you will have to either install mailwizz in a subdomain for b.com domain like email.b.com or you can still use the install from email.a.com but use a tracking domain for b.com domain. A tracking domain is simply a CNAME(DNS Record). In our case, we define a cname for b.com domain, namely email.b.com that will point to email.a.com subdomain. Once we do this, propagation can take up to 48 hours, so after 48 hours, we can go in the backend of mailwizz website and add a new tracking domain, the email.b.com domain. Once we did this, to check if everything works properly, we have to access http://email.b.com and we should get same page that we get when accesing http://email.a.com. If it's not showing same page, then you don't use a dedicated ip address for a.com domain and you should. Now, our tracking domain is in place, all we have to do si to associate it with a delivery server and all emails that will be sent via that delivery server will change all the links from email.a.com into email.b.com. SENDING DOMAINS: You should use sending domains only if your delivery servers do not sign your emails with DKIM, because the only purpose of sending domains is to identify the domain from the "FROM" or "Return-Path" of your headers and generate a dkim signature for them. Like in the above case, if your domain is b.com and you send emails from firstname.lastname@example.org, then you can add a sending domain (b.com) for which mailwizz will generate the dkim records (you have to add the public key into your dns records, as a TXT record). Once the domain is confirmed, each time when mailwizz finds an email that contains the domain b.com in it's FROM/Return-Path headers it will append the right DKIM signature.