Set up email on Mac running High Sierra

Add or remove email accounts in Mail on Mac - Apple Support describes the fundamental steps.

You will notice they are for macOS 10.14 "Mojave" which are the closest instructions that remain available from Apple. However, I happen to know they are unchanged from High Sierra 10.13, and should work. Follow those instructions to the best of your ability.

That's the good news. The bad news is that High Sierra 10.13 is quite old at this point, and your Mac's Mail client app may not be able to cope with newer email service protocols. For example High Sierra has not been able to cope with Microsoft Exchange email services for quite some time now, but iCloud Mail should work.

If you use some other, non-Apple email service, you may still be able to use it with your email service provider though, so try it and let us know if you get stuck.

John Galt wrote:

…The bad news is that High Sierra 10.13 is quite old at this point, and your Mac's Mail client app may not be able to cope with newer email service protocols. For example High Sierra has not been able to cope with Microsoft Exchange email services for quite some time now, but iCloud Mail should work.

Correct. macOS 10.13 will not be able to cope with newer security certificates that can be presented by mail servers and web servers.

This stuff: Updating the Certificate Trust Store on m… - Apple Community

MrHoffman perhaps you could offer some help here: Safari won’t show details of unsecure website - Apple Community

I managed to fix a printer whose certificate had expired but I don't recall exactly what I did. That NAS is probably suffering a similar fate.

John Galt wrote:

MrHoffman perhaps you could offer some help here: Safari won’t show details of unsecure website - Apple Community

I managed to fix a printer whose certificate had expired but I don't recall exactly what I did. That NAS is probably suffering a similar fate.

I verified the certificate override path works as expected on iPadOS 26.2.1.

We’ll see what path that mystery NAS case might follow.

You're welcome. Pardon me for having sidetracked your question with a request for help.

Were you able to successfully add any email accounts to Mail? A workaround might be to use a webmail client, assuming your email service provider provides one.

High Sierra is rapidly becoming obsolete, if it's not completely obsolete already. Even if a webmail client is an option, there might not be any browsers that would work with it anyway.