Divorce letter not reaching the wife

I wrote a divorce letter to my wife and signed it in the presence of a qazi and two persons. The qazi was about to send the letter within the next 3 days. I did it because she was caught red handed talking closely to a stranger over the phone. She was pregnant at that moment.

After two days, I thought I will give her one more chance to correct herself and stopped the letter and the letter was destroyed. I also informed the two persons that were present when I signed the letter and I started living with her. She gave birth to a baby girl one month ago.

I thought it was necessary to deliver the letter to her for the divorce to be effective. She still does not know about the letter.

Recently I asked an Imaam about it and he said that the divorce has occured even though my wife was not present and the evidence was destroyed.

My question is, is my marriage still valid?