Dear Othelo.
I write this letter as a friend who cares and admires the man you are. What I am going to talk about here may be received with disbelief, but I ask that you read it carefully and believe everything that is written.
Iago is not your friend. He is very envious of everything you have achieved and is trying, at all costs, to make you lose all the good that you have and live in disgrace and shame.
Iago is trying to manipulate you against your wife. You know the woman you married and you don't have to think too hard to realize that she would never commit adultery, much less with Cassio who is the person who most admires and respects you. Desdemona has rejected her own family for you, which shows that you are very valuable to her and she does not want to lose you or disappoint you.
Iago knows how an adulterer destabilizes you and that's why he invented that Desdemona is unfaithful, in addition to creating lying evidence for you to believe him. If you interrogate his wife, you will be able to confirm everything I am talking about.
It is important that you get away from Iago and listen to the people who really have you as something important for their lives. Do this before it's too late.
Graciously.
A friend.