Why do ESPs recommend using custom sending domains?

A custom sending domain means using something like mail.yourbrand.com as the technical sender of your email, instead of relying on your ESP's shared default like mailer.esp.com. The recommendation is universal because it affects reputation, authentication, and your long-term portability.

First, reputation. Mailbox providers track sender reputation at the domain level, not just the IP level. When you send on your own domain, every good sending behaviour (engaged recipients, low complaints, authenticated mail) accrues to your brand. When you send on a shared ESP default, you're inheriting the reputation of every other customer using it. One bad neighbour can drag you down, and none of the reputation you build is actually yours if you ever switch ESPs.

Second, authentication alignment. Modern deliverability requires your DKIM signing domain to align with the visible From domain, and your DMARC policy to enforce that alignment. The 2024 Gmail and Yahoo bulk sender requirements made this a hard line. If you're sending from hello@yourbrand.com but your DKIM signature is on esp.com, alignment fails and DMARC evaluation cares. A custom sending domain makes alignment trivial to set up correctly.

Third, portability. Your domain is yours. If you migrate from one ESP to another, your domain reputation travels with you, your DNS records are already set up, and your recipients don't notice the change. On a shared ESP default, you start over from scratch.

Setup is usually straightforward. The ESP gives you a handful of DNS records to add (CNAMEs or TXT records for DKIM, a subdomain delegation, maybe a custom return-path). You add them at your DNS provider. Once propagated, your ESP is signing and returning bounces on your domain, and you own the reputation that builds from there.

So if If you want to check that your custom domain is aligned correctly once it's live, the DMARC checker at Review My Emails will tell you in a few seconds.