Sometime emails will appear being delivered successfully but just not show up in the recipients’ inbox. The first thing to check here (after making sure the email address is correct) is if the recipient has any rules set in their inbox which has moved the message into some folder or other.
Next it’s best to check the recipients’ Spam or Junk folder to see if the message has ended up in there. If so, then sending them a Whitelisting kit for your group will be the best course of action.
Even if the messages aren’t showing up in their Spam or Junk folder then sending a Whitelisting kit is a good thing to do since the messages could just be silently being rejected as spam.