Access to the Montreal (YUL) International Maple Leaf Lounge™ (MLL)

Latest news, as per The Altitude Report, "October Edition", sent on November 1, 2017:

Our Montreal International Maple Leaf Lounge™ next to Gate 52 offers a peaceful, sophisticated place to relax before taking flight. As a friendly reminder, eligible customers who are departing on an international flight may access the lounge, regardless of the departure gate. Those departing on a domestic flight may only access the international lounge if their departure gate is between 50 and 68. They can also access the domestic Maple Leaf Lounge next to Gates 1 and 3.

An aside from yyznomad: (November 1, 2017)

From my own insider information, there was a recent meeting to discuss access to the YUL International MLL. It was to discuss access eligibility for domestic flights, due to overcrowding. I found this out due to an experience where I was flying out of Gate 50 on a domestic flight and was denied by both a lounge agent and a concierge seated next to him. After patiently waiting for them to reconfirm the rules, only upon my insistence, did they let me in. The agent vehemently advised that it was Gate 52 and higher and told me to talk to the concierge next to him. The concierge then asked the same agent what the rule was, and sided with the agent without verifying. Two days later, another agent apologized on their behalf for getting it wrong as it was still Gate 50 to 68. In any case, the rule still stands for gates 50-68.

Also, if you got in, but as an exception to the rules, then consider yourself simply as lucky.

So don't be afraid to stand your ground, with or without printed proof. So as long as you are eligible, stand your ground! You are most likely to get an agent who won't know the rules and wrongly turn you away.