By having a common email address that you give out to each service you sign up on you make it easier for them to aggregate the data and build a more detailed profile on you, in order to avoid it you would use email aliases (dummy address that serve the purpose of only forwarding emails they receive from and to one of your real address). If you use a custom domain name you can potentially create an infinite amount of them, but you expose yourself to being tracked anyway because they would all have the domain name in common e.g. a@mydomain.me, b@mydomain.me, etc. and they would notice that it all comes from one user for service, so it's easy to guess it is actually just one real person.
To avoid that happening, you would have to use a public aliasing service so you can blend in with the other users