I need to travel from Montreal to Atlanta. I have seen two flights on the 5th August (flight 3502 and flight 1217). However, the time to change planes is very reduced (45 minutes). In Montreal, I will pass customs, so they would be like a domestic flights. Are 45 minutes enough? Would I need to pick up the luggage and check-in it again?
I don't quite have enough information because you didn't mention which airline you are on (I would guess UA but UA 1217 is from LAX?). If there is no terminal change, this is fine.
If this is on one ticket, your luggage will be checked through to the final destination from YUL and there is no further security check; 45 minutes is sufficient.