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

"What happened?" she asked.

Son in law: "What happened? - I'll tell you what happenned! I sent an email to Mary saying that I was coming home from my
trip today. I got home and guess what I found?....... My wife, yes your daughter, 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. Mary would never do such a thing! Wait a minute while I check what happened."

Moments later she comes back with a big smile.
"You see, I told you there must be a simple explanation..............

Mary didn't receive your email!!"

I had a mother-in-law like that a long time ago.

It all makes sense now...