I find that quite frequently Emails that I send do not reach their destination. When this happens there is no notification or evidence that they have failed to transmit to the recipient. The problem can occur to some recipients but not occur at other times. If I send to multiple recipients sometimes some recipients receive the Emails, others do not.. I am not suggesting that it is TPG's fault but wonder if you may nave any idea as to why this may be occurring. I changed to version 10 of Windows recently & this problem only became apparent after that time.
Did you receive any bounce back error message to confirm that your emails weren't sent to your recipients?
If none, then you may advise your recipients to check if the emails were directed to their spam/junk folder.
If it's not there, then your recipient may need to raise the issue with their email admin.