That's difficult, as no error messages are available. One possible reason might be sending emails with attachments where the receiving site prevents it. Another possibility is that security for email has been increasing during past years and some of the receiving servers may be rejecting your message. My first suggestion would be to identify, where possible, which email providers are not letting your messages through. My second suggestion is to review your SMTP settings to see if you are using passwords or OAUTH2, and security such as SSL/TLS or STARTTLS .

Hi David, Many thanks for your reply , Im not using passwords and security is set with SSL/TLS At a total loss with a limited knowledge of computers too

Maybe if your post a screenshot of your SMTP server settings, some suggestions may surface. I am not saying that is the problem, but it might be. A screenshot of your account email settings may also prove useful.