William has a Samsung Phone and an Apple computer. He's having issues with his Mac getting his text messages, but his phone doesn't get them. Leo says that if you had an iPhone previously, the message could have been sent to your Apple ID account via Apple Messages instead of sending via SMS text messaging. So the people are texting you and selecting your email, which is in blue, because they may have an iPhone. The only place the email will show up is in Messages on your Mac. It's very confusing when people use multiple phone platforms and Apple won't release an Android version for Messages.
William is also getting email messages that just say "Null." What is that? Leo says Null means nothing, and so William is getting empty email messages with no content. That sounds like a bug, where the email app isn't seeing a message and just putting null in the body of a message that isn't there. Leo also says that Samsung may be sending the email to both your email account and your phone, and the phone is getting the null message.