We changed to ProtonMail.
ProtonMail works fine on our iMac and is easier to use than Tutanota. There seem to be no limits on daily number of emails, a Tutanota problem as every cc or bcc is counted as a separate email by Tutanota.
It was fairly easy to move our contact list over.
Mrs C spent considerable time contacting others to let them know of the change and cut and pasting emails to save them in .doc or .txt format in folders on our Macs.
We are reasonably happy with the level of security as we use a good VPN, which does not keep records, and a good anti-virus/malware that receives updates every day on both Macs. Time Machine backups are done every hour and we have four other portable hard drives which we rotate for backups every week. Two of these are kept well away in case of flood, fire, burglary, etc. We use quite long and complex pass phrases.
The MBA is current for macOS and Firefox and we recognise we will eventually have to replace the iMac.
Thanks all for the comments.