Do you get a copy of the emails sent in the folder that you have designated to store sent emails? If so, TB will be sending them to the SMTP (Outgoing) Server and receipt will have been acknowledged.

The problem would, therefore, likely be something that's happening after the email has been sent which is out of TB's control.

Do the missing emails to your son end up in the Junk or Trash folder in the online account associated with your son's email address?

Thank you for your reply. The copies of the sent emails are in the sent folder. At first some emails went to his Spam folder then later he did not receive them at all and a week later they were returned to me as undelivered. I understand that Yahoo have blocked Tiscali because of no DMARC/DKIM. I have added DKIM to incoming emails but cannot find how to add it to outgoing emails. I think this may be a Thunderbird problem.

See david's response here - https://support.mozilla.org/en-US/questions/1421742 It would not be a Thunderbird problem.