Is it permissible to greet my mother's maternal aunt with "Salam"?

Your mother's maternal aunt (khālat wālidatik) is your maternal aunt (khālah), and she is a maḥram to you, so it is permissible to shake her hand. The verse (Forbidden to you are your mothers, your daughters, your sisters, your paternal aunts, and your maternal aunts) includes the maternal aunt of one's father, mother, grandfathers, and grandmothers. A maternal aunt (khālah) is any sister of your mother or grandmother, no matter how distant the lineage, and similarly for a paternal aunt (ammah).