Why do ESPs recommend using custom sending domains?

Using your own sending domain means reputation accrues to your brand rather than the ESP's shared infrastructure. When you send from mail.yourdomain.com instead of a generic ESP domain, mailbox providers evaluate your domain's history, building a reputation asset you control.

Custom domains enable proper DKIM and DMARC alignment. Authentication passes when your sending domain matches your visible From domain. This alignment is increasingly required; the Gmail and Yahoo 2024 requirements mandate it for bulk senders.

ESP default domains are shared across many customers. Problems caused by other senders can affect your deliverability. Custom domains isolate you from that risk while establishing your brand identity in the inbox.

Fly your own flag. Ships sailing under a shared banner share its reputation, good or bad.