why are messages from my powerbook intermittently not delivered to one specific contact?

Hi there mpenna166,

Thanks for using Apple Support Communities. It sounds like messages to one specific contact are not being delivered sometimes. Since this is happening with a specific contact, it may be an issue on their end. For example, if they are not in an area with Wi-Fi or mobile coverage when your message is sent or if they are having an issue receiving messages. To reduce the chances of this happening, you can set up Text Message Forwarding so you can send and receive SMS messages with the Mac when iMessage is not available. Learn more here: How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac.

We hope this helps!