Strange email problem in MacOS

[#1]

I'm trying and failing to set up my Plusnet email on a new MacBook.  I have another Mac on which the account works fine.  Both are running the same version of MacOS (10.13.4), both are running the MacOS email client.  Both have exactly the same account settings (verified multiple times - I even changed the password to be 100% I was typing in the correct one).  But the MacBook says "unable to verify account name or password".  The same settings work fine on my phone and iPad as well.

Help!

[#2]

There is another topic covering this problem https://community.plus.net/t5/Email/IMAP-works-on-some-devices-not-other-with-exact-same-settings/m-... which unfortunately never reached resolution. We suspect Apple Mail on High Sierra no longer allows new accounts to be set up using an insecure (non SSL/TLS) connection. However those previously set up continue to work.

However it would be useful if you could work through what was suggested in that topic and see if anything help..

[#3]

That's weird.  I'll have a look, thanks.

[#4]

Tried and failed.  It is one of my 2 regular-use email accounts which is a right pain - I can access via the Plusnet website but it's much less convenient than having it in the mail client.

[#5]

Are you able to try an alternative email client?