Text messaging from iMac

I just had someone here yesterday that is a computer tech but I'm still having the same problem. Sometimes when I text from my iMac, it fails to send. Yet, when I send from my iPhone, no problem. He checked my settings and said they seem to be in order. I've even gone so far as to log out of Messenger and log back in and try to send but I still get the same message Failed to Deliver. I've logged in and out and synced but still to no avail. On looking more closely, it appears to be when trying to text as MMS. Those don't go through. iPhone to iPhone Fails to Deliver. However, if I send a message to both Android and Apple iPhone, it goes through as SMS and both parties receive the message. Has anyone had this problem. I'm not a techie so if you can help, please bear that in mind. Thanks so much.