Some friends did not receive some of my email sent to them but could receive mails from my singnet a/c. Why?

This has been happening in the past weeks and it seems that most of the receipient who did not receive my mails are using hotmail a/c. There's no warning that the email was undeliverable. I was verbally informed by my friends. It gives me second thought on yahoo mails' reliability.
Some friends did not receive some of my email sent to them but could receive mails from my singnet a/c. Why?
It may be stored in their bulk/spam folder. Check for those messages in spam/bulk folder.





ask them, to Add your emails to address book, in order to receive in inbox.





ask them to open bulk folder, and open your email in bulk folder, and click "not spam"