Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
A 'Mahram' (unmarriageable person) is a person (male) with whom marriage is forbidden forever. Such a person can see, stay in seclusion, shake hands, travel, etc. with the woman for whom he is 'Mahram'. There are two exceptions for this ruling:
A) The woman after having separated through 'li'an' becomes forbidden forever for her husband, and alien to him, the rulings of 'Mahram' do not apply to her.
B) A wife is not among 'Mahaarem' for her husband but all the above rulings apply to a husband.
Allah knows best.