This error came about due to changes at the Internet Service Providers' mail servers when security was updated in 2017. We produced a new external mailer for the old Professional program to resolve the problem.
Make sure you are on the latest version of the Professional program by applying the update which can be found in the article Professional Update & Help Files
Then under email settings choose 'External engine' and fill in the smtp, mode and port boxes according to info from your service provider or which you may find if you have Outlook settings entered. Usually it will be port 587 with SSL or port 465 with TLS but you can try these settings till you get it right.
Note that Antivirus and firewall settings may also need to change.