These addresses do not build up sender reputation in the same way as a domain that is tied to a unique domain + mail server since a part of what ESPs note when determining sender reputation is past sender history and how this engagement was received in association with the domain, the mail server, and the email address in question.

Since free email addresses are more prone to abuse, the sender's reputation does not resolve back in the same way as an email address that is tied back to a domain so they do not receive the benefits of inbox warming in the same manner.

For inbox warming to be effective there needs to be a unique domain associated with the email address in question performing email activity so email addresses under free services do not directly benefit from building up sender history.

We´d recommend creating an email address under Google Workspace (or any other paid domain with paid hosting) that´d point to your domain website.

It´s not a free @gmail address, so you need to pay for it, however, this would build your positive reputation in around 45-60 days.