Not all my e-mails are being received even though they get “Sent” fine on my end

I send emails from @hotmail.com account.  All emails go through fine on my end and they appear in my Sent folder.  However, some but not all recipients tell me they never received my email.  These recipients receive my emails sometimes but not others.  So, I never know which of my Sent emails actually made it to the recipient or not.  I am not on their blocked or junk list since some of my emails get received and some don't.