BIMI
BIMI lets a domain publish a logo that participating mailbox providers may display beside authenticated mail. It requires DMARC enforcement first.
What BIMI is
BIMI — Brand Indicators for Message Identification — is a TXT record at default._bimi.example.com pointing at a logo file, and optionally at a certificate attesting that the logo belongs to you.
Participating mailbox providers may display that logo next to messages from the domain. Whether they do, and where, is entirely their decision.
What a BIMI record looks like
The record itself is a version tag and two URLs: default._bimi.example.com. 3600 IN TXT "v=BIMI1; l=https://example.com/logo.svg; a=https://example.com/vmc.pem".
The l tag points at the logo and the a tag at the certificate asserting the right to use it. A record may publish l alone, though mailbox providers differ on what they will display without the assertion.
BIMI is specified in an IETF draft rather than a published standard, so what a given mailbox provider requires is closer to a vendor behaviour than a settled rule.
DMARC enforcement is a prerequisite
BIMI only applies to mail that passes DMARC, and the domain must publish an enforcing DMARC policy — quarantine or reject, not none.
This ordering is the point: the logo is a reward for authenticated mail, not a decoration that can be attached to anything.
The logo and the certificate
The logo must be SVG in a restricted profile: square, with no scripting or external references. Ordinary SVG exported from a design tool will usually be rejected.
Several providers additionally require a Verified Mark Certificate, issued by a certificate authority after checking that you hold a trademark on the logo. That is a commercial process with real cost and lead time.