Yes, the version of Mail is too old, which is tied to the OS version.
What is OAuth?
OAuth is an open-standard authorization protocol or framework that provides applications the ability for “secure designated access.” For example, you can tell Facebook that it’s OK for ESPN.com to access your profile or post updates to your timeline without having to give ESPN your Facebook password. This minimizes risk in a major way: In the event ESPN suffers a breach, your Facebook password remains safe.
OAuth doesn’t share password data but instead uses authorization tokens to prove an identity between consumers and service providers. OAuth is an authentication protocol that allows you to approve one application interacting with another on your behalf without giving away your password.
In the interim there's WebMail, then finding a different Mail client for 10.13, perhaps Thunderbird?
Thunderbird — Make Email Easier. — Thunderbird