I found a thread on a similar question where someone posted that there is problem between gmail and apple mail since the latest HS update - he stated:
" Google deemed Apple's Mail client in (High) Sierra as "unsafe". If you have "restrict access by less safe applications" switched on in your GMail account, Apple Mail will not be able to login. The error as described in the question above will then appear.
The solution is to either 1) use a different mail client or 2) enable access in your Gmail account. Here's how:
- open https://myaccount.google.com/lesssecureapps in Safari and logon if requested
- flick the switch to the "on" position
- start Apple Mail and try to sync your GMail account (this should now work)"
Although I had checked into this before, because I have 2-step verification enabled, it said this wouldn't work. This time I tried it - requested an app-specific password. After some trial and error (couldn't get Mail to set up a pop account again- it would only do imap), I finally got everything set up properly. Whew! Such an easy solution - 2 days and countless pointless changes to my os.....
Thank you for your all your help.