It is rightful for men to be watchful of their wives but not to an extent of suspicion and distrust. Some men are suspicious and doubt their wives' faithfulness. This is dangerous and makes life very difficult for all the family.
A man who is suffering, from this behavior, constantly picks up fault with his wife. He monitors her closely and follows her everywhere. He finds supporting evidence for his cause of suspicion from every thing. If he sees his wife talking to a man, or finds a photograph of a man among her belongings, or finds a letter written to her by a man, or finds a man looking at her, he would reach certainty with respect to her unfaithfulness. If his wife hides a letter from him, he would think that it is a love letter. If she expresses her love less than before, he would doubt her sincerity.
He might even think that since his daughter does not look like him, his wife must have committed adultery.
All such examples can be regarded as a firm proof of a woman's infidelity by the suspicious husband. The situation becomes worse if a relative or friend agrees with his suspicion.
Families, who are affected by this illness, suffer a great deal. The man would act like a detective around the house, and his wife would feel as if she was being kept in custody. They would both suffer mentally and their marriage would become endangered. They might even resort to divorce or murder.
There are many cases of homicide and suicide which have occurred as a result of suspicion.
Under these conditions a man and his wife should be aware of the possible grave consequences and, through wisdom and understanding, repel any danger which could threaten their marriage or even their lives. They only need to be a ware of the potential danger and be able to think clearly in order to overcome their problems.
A man should give up his fanaticism and extreme jealousy. He must act logically. He must be aware that convicting his wife of adultery is not a trivial matter, and that such an allegation needs definite proof.
Allah states in the Holy Qur'an: " O you who believe! avoid most of suspicion, for surely suspicion in some cases is a sin..." (49:12)
"The Prophet (SA) of Allah stated: 'Whoever, falsely, accuses his wife of committing adultery would lose all the benefits from his good deeds just as a snake sheds its skin. And for each hair on his body, one thousand sins would be written down in his record (for the Day of Judgment)' 
"The Prophet (SA) of Allah also stated: 'Whoever falsely accuses a faithful man or woman, Allah would, on the Day of Judgment, hold him on a heap of fire so that he receives the punishment for his sin'."
As long as a woman's unfaithfulness is not proved through firm evidence, a man does not have any right to accuse her, otherwise he would be committing a sin for which, as per Islam, he is to be punished with eighty lashes.
Evidence based on purely imaginative grounds are not indicative of any thing. Old letters, pictures, etc do not prove any thing.
Keeping such things is not right, but this is a mistake that most young people commit and it is not a thing to be seriously concerned with.
If a woman is seen conversing with a stranger, although she is not right in doing so, this isolated event cannot be held against her as a proof of her unfaithfulness. This is because she might have thought it rude not to pay heed to that man, or he might have not been a stranger but a friend of her father or brother.
If a woman makes a compliment to a man, although she should not, it may be out of simplicity and therefore cannot be indicative of her unfaithfulness.
If a woman tells a lie about a relationship, or hides her letters, it may be that there is a good reason for it or she may fear her husband's groundless accusations.
If a woman has grown cold towards her husband, it may be that she is upset with him, that she may be ill, or may have other problems.
In brief, for all situations which may indicate symptoms of unfaithfulness, one can find tens of good reasons which render the possibility of any wrong doing as void.
Dear Sir! for the sake of Allah stop being suspicious. Consider yourself a fair judge and view the problem with logic. Measure the degree of the possibility of your wife's unfaithfulness and find out whether it is definite, just a suspicion, or even feasible?
I am not saying that you should be indifferent or careless but that you should act upon the amount of evidence you possess and not more. Why should you exaggerate the problem with baseless suspicion and make life difficult for yourself and your family? How would you feel if anybody accused you in a similar manner? Why do you think in an unfair manner? Why should you disgrace yourself and your wife? Why can you not have mercy upon your wife? Have you ever thought that she might eventually deviate from the right path through your mistrust and false accusations?
"Imam Ali (AS) stated to his son, Imam Hasan (AS): 'Be careful not to act possessively when you should not. Because this would incline the right people towards corruption and the chaste people towards committing sin'."
If you are suspicious of your wife, do not discuss it with just anyone whom you see, because they might approve of your suspicion out of enmity, simplicity or carelessness. They might approve of your suspicion out of enmity, simplicity or carelessness. They might even strengthen your suspicion and cause you unhappiness in this world and the life after. You should not, specifically inform your mother or sister because they would naturally be in agreement with you and thus increase your suspicion. You must seek advice from wise and experienced friends or relatives.
The best approach, however, is for you to talk to your wife and ask her for an explanation. But you should not seek to prove her guilt. Listen to what she has to tell you and decide like a fair judge who is free from any prejudice.
At least try to believe her and imagine your brother-in-law is presenting you with evidence of your sister's unfaithfulness. Why should you act mercilessly and regard her as a proven criminal'?
Be wise and patient, lest you divorce her on baseless grounds. Suppose you endure the sufferings of a divorce, but how certain are you about the next wife? You would still be suspicious. What is their fault if it is you that is suffering with this illness? Be wise and try to understand your own problem.
Be careful not to resort to commit suicide or murder your wife. Because you would destroy your life here in this world and the Almighty Allah would punish you in the life Hereafter.
You must know that spilling blood would one day be revealed and then you would either be executed or would spend your life in prison.
If you do not agree with this point, then just take a look at the statistics of convicts.
Wives of suspicious men also have a great responsibility with respect to their families. These women must sacrifice and prove their ability in such difficult situations.
Dear madam! first of all your husband is infected with a dangerous illness where he, unwillingly, takes the wrong steps which would endanger your family.
You must express your love for him as much as possible. He must be certain that he is the only man in your life. Be patient with him, do not shout at him, do not refuse to talk to him and do not be stubborn with him.
If you feel that he is monitoring your letters or controlling your comings and goings, do not protest. Tell him every thing, tell the truth. A void lies or denials of the events which have happened. If he ever finds out that you have been lying about anything, he would regard it as a proof of your infidelity, the damage of which cannot easily be repaired.
If your suspicious husband ever asks you not to associate with a certain person or wants you to do a certain task, then accept his word, otherwise the cause of his doubt in you would strengthen. In brief, avoid all deeds which could make him suspicious of you.
"Imam Ali (AS) stated: 'Whoever exposes himself to accusation, must not blame those who become suspicious of him'."
If your husband shows animosity towards any one, then you should break your contact with that person altogether.
Dear madam! it is better for you to keep your family together than keeping your friendship with others. Do not think that you are a slave in the chains of your husband, but realize that you are an ill man's wife.Remember when you made a marital covenant with your husband, you undertook to share all the happy and the sad occasions of life. Is it now fair for you to mistreat your husband who is suffering from an illness? Put away immature thoughts and be provident. By Allah, however much your sacrifice for your family, it is worth it. A good woman is one who can cope with in difficult situations.
"Imam Sadiq (AS) stated: 'Jihad of woman is in her patience towards her husband's malicious acts and fervour'." 
Do not do anything which would make your husband suspicious. Do not look at other men.
"The Prophet (SA) of Allah stated: ' Allah would be greatly angry with a married woman who fills her eyes with the looks of strangers'." 
Do not associate with strange men. Do not leave the house without your husband's permission. Do not get in strangers' cars. Only your chastity is not enough: but you should seriously avoid anything which would arouse your husband's suspicion. He might become suspicious of the most trivial points of your behaviour.
"A 27-year old woman said in the court: 'It was the winter of 1963, when on a snowy day, I got in a car which belonged to my friend's uncle. She offered that her uncle would give me a lift home. I accepted and got in his car. When we reached home, my husband was standing by the door of the house and since I did not want him to see me in another man's car, I asked my friend's uncle to drive on, which he did. Later, my husband who had seen me in that car, asked about it, but I denied everything. He became more suspicious and it reached a stage where he did not even believe my friend's witness. Now it is eight years that he has neither lived with me nor divorced me. I do not know what to do'." 
Who do you think is to blame in this story?
I would say that the woman is more guilty than her husband. It is she who, out of carelessness and simplicity, had placed herself and her husband in this situation.
First of all she should not have accepted a lift from a stranger as this is not a correct thing to do for any woman. It is not proper and it may be dangerous.
Secondly, she should not have acted the way she did when she saw her husband. She should have stopped the car and should have explained to her husband.
Thirdly, one of her mistakes was to tell the driver to drive on.
Fourthly, she should not have denied it later. She could have explained every thing even at this late stage and it could have helped to solve the problem.
Of course the man is not right either. He must not regard this event as a conclusive evidence of his wife's guilt. He must consider the possibility that his wife might have carelessly got in a stranger's car and then she might have, fearfully, asked the driver not to stop and naturally denied the whole affair.
He must investigate the matter and once he is certain that she is not guilty, he must be forgiving.
 Bihar al-Anw'ar, vol 103, p 248.
 Ibid, vol 75, p 194.
 Ibid, vol 103, p 252.
 Ibid, vol 74, p 187.
 Wasa'il al-Shiah, vol 14, p III.
 Bihar al-Anwar, vol 104, p 39.
 Ittela'at, 23rd Esfand, 1350 Solar Hijri.