Issues with adding external internet account such as email to MacOS High Sierra

Open Keychain Access in Utilities, then either check the Password under that item, change it, or delete it and start over.

Enter the part after the @sign in the search bar.

You may have duplicate or more entries in Keychain Access.

If using a browser to login via WebMail works it's not Name or Password, but likely old passwords interfering.

There''ll be at least 2 entries, maybe more.

The trouble is that Mac Mail almost always chooses the old Passwords if you don't change or get rid of the old ones.

Start over with Mail…

Quit Mail.

In the Finder, hold down the option key and select

Go ▹ Library

from the menu bar. Move the following items from the folder that opens to the Trash (some may not exist):

• Caches/com.apple.mail

• Saved Application State/com.apple.mail.savedState

• Application Support/AddressBook/MailRecents-v4.abcdmr

• Containers/com.apple.mail

• Mail

• Preferences/com.apple.mail.plist

• Preferences/com.apple.mail.searchhistory.plist

Note: you are not moving the Mail application. You’re moving a folder named “Mail.”

Try setting up mail in a different login? See if there’s something local to this login/

Tried using an application-specific password?

Any add-on anti-virus or add-on anti-malware, any add-on cleaners, or any add-on firewall-filtering tools? Remove per the vendors’ instructions and reboot and test, if so.

Google has a habit of some odd implementations over the years, too.

Previous discussions abound: High Sierra gmail problem - Apple Community

Troubleshoot email problems in Mail on Mac - Apple Support