The mother-in-law comes home and finds her son-in-law furious and packing
his suitcase. "What happened?"

"What happened? - I'll tell you what  happenned! I sent an email to my wife saying that I was coming home from my
trip today. I got home and guess what I found?... My wife, yes my Rachel, with a naked guy in our marital bed! This is the end of our marriage, I will leave forever!"

"Calm down!" says mother-in-law. "There is something odd about this story. Rachel would never do such a thing! Wait a minute while I
check what happened."

Moments later, mother-in-law comes back with a big smile. "You see, I said there must be a simple explanation..."

"Rachel didn't receive your email"