I just want to know how to fix my current problem.

Update2: After looking through imessage on my mac I realized that all the contacts that I cannot send to have usually been sent through text not imessage since their text bubbles are green. These contacts are people who don't have this red highlight on their names and if they do have their name highlighted in red then I cannot send messages to them. Update: After playing around I realized that I can send messages to some of my contacts. It was either delete some files through Keychain Access, restart something or turn on the message forwarding. Going through pages of forums many of the problems were either a year old or older and were very repetitive. Due to frequent queries, we came up with some fixes with tutorials to resolve this issue. Actually, many people found their iPhone messages or iMessages are not delivered or received with the update of the iOS 12. This time again the complaints came with iOS 12 as well. About a week ago suddenly I couldn't send any messages without getting the 'Not Delivered' and that red exclamation mark. iMessage is a free messaging service provided by Apple, but sometimes it fails to work after new iOS installation. At first iMessage worked for a decent period of time I was getting texts on time and sending them through my Mac just fine.

Recently I changed the storage to an SSD and with that change I started to set up iMessage. I have an iPhone 5s and a MacBook Pro 13' mid 2012 model with OSX 10.10.4.

Messages not delivered mac