Unexpectedly cannot send emails from Apple Mail for waitrose account supplied by Plusnet

[#1]

For no apparent reason emails are no longer being sent from Apple Mail

I have been using High Sierra for years but with no issues like this

Message says "Cannot send message using the server home email" and "An SSL error has occurred and a secure connection to the server cannot be made"

I suspect the settings for the Outgoing Mail Server (SMTP) need reconfiguring at least

The current server settings (which have worked for years) have Description: home email; User Name: [optional]; Password: [optional]; Host Name: smtpmail.waitrose.com

I have tried various configurations e.g. putting in User Name and Password (as logging on to webmail); or changing Host Name to mail.waitrose.com or smtpmail.greenbee.net or mail.greenbee.net -- all to no avail

Any assistance welcome!

Thanks

[#2]

It occurs to me that email first failed to send just after the clocks went back. Does this ring any bells?