configuring unibox

Trying to configure the Unibox email client with my iCloud account (which I have done once before successfully so I know it can work).

Failing consistently on the Outgoing Mail Server page of the Unibox config wizard with the following error:

The operation couldn't be completed: kCFErrorDomainCFNetwork error 1

I'm supplying the documented SMTP server parameters from this page:

https://support.apple.com/en-us/HT202304

server: smtp.mail.me.com

Connection: STARTTLS

Port: 587

Username: <my complete iCloud email address>

Authentication Method: Autodetect

Password: <providing an app-specific password which I just created>

Thanks in advance for any help. I'm going to all this trouble because I really like the Unibox Inbox filter which groups all messages from a given sender together (like text messages) to allow you to keep control of your Inbox better.