Why use a sending subdomain?

A separate subdomain for your automated mail protects your main domain. It is one of the few measures that costs almost nothing and prevents a lot.

What it does

Reputation is built on the subdomain, not on your main domain. So a problem with one campaign does not affect your ordinary mail between colleagues and customers.

How it works in practice

You send from an address on that subdomain and configure authentication there. The recipient sees your brand, with a small difference in the address.

Separate by type as well

One subdomain for transactional and another for marketing. The two reputations then stay independent.

Plan for it

Allow time to build the new subdomain's reputation. Switching on a Monday and sending a big campaign on Tuesday is exactly what not to do.

What it does not fix

A subdomain does not repair a bad list. If your complaints come from the content or the recipients, you have only moved the problem.