SMTP proxy will have that trust for you and help you on setting up things at the domain level like reverse DNS, DKIM, etc., that are needed to combat forged “from” addresses and many selfhosters miss.

Only difference is you need a full email stack on your side. Only SMTP traffic (in and out) would go through the proxy and to your local SMTP server. It solves the IP trust issue in many cases (though not all as some places do block the proxies on purpose as well), but nothing can really solve the domain trust issues when they come up except directly contacting the companies who have those policies and often resolving it isn’t really possible since they often don’t have external tech support for that stuff. It’s so annoying when they only whitelist gmail, outlook, and a few other big company domains.

There are then various ways to view the emails and compose and send. If you want to have clients accessing the email via IMAP/POP then you need that server in addition to the SMTP server, but all of that traffic could then be kept local. If you want to only have a web client there are clients that access the email directly without need for IMAP/POP.

But this way, although your email does pass through the proxy so you need to trust them, at least the data at rest is not stored there which increase the likelihood they’ll change policies on reading your email like gmail did, or charge you for storage, which is expensive these days.

Personally I use mailcow which is a pretty nice stack, but it would probably be overkill for many people. And I have a good understanding of the issues with both IP and domain white-/black-listing. So it works decently for my needs.