TLS
Encryption in transit — mail is sent over a secure connection between servers.
Say it out loud
“The email is encrypted while it travels between servers. Our platform handles it.”
TLS (Transport Layer Security) encrypts the hop between mail servers. Gmail's sender guidelines expect it for mail sent to Gmail. Every mainstream platform does this and has for years; it only becomes your problem if somebody built a custom sending setup.
ESMTPS — the S is the whole tell. Plain ESMTP means it travelled unencrypted.
Your platform
Klaviyo, Mailchimp and friends do this automatically.
What goes wrong
Nothing, on a mainstream platform. On a self-hosted or legacy MTA, an expired certificate silently downgrades connections and Gmail starts refusing mail.
The dated rule behind this
A definition is not a citation. These are the pages with the primary source, the date it was published, and what to do about it.
See also
Where this sits
Stop 3, it leaves your building. Your platform hands the message to a mail server, which stamps it with proof of who sent it.