ALL not ignoring date filter

[Paul Brown]

Good evening from Spain!

I am hoping to get some advice since I am stuck at a problem and can't seem to find a solution. I have a very simple model consisting of a fact table (emails sent: date sent, lead, category...), a calendar table and a Leads lookup table (referenced to the fact table.
This is a screenshot of the model setup:

What I'm trying to achieve is the minimum date a lead was emailed. This is simple enough against a lead, but what I'm after is to identify which month the lead was first emailed (in order to then count how many leads were first emailed by month). I have set up a matrix with (from calendar) Year, Month Name and (from Leads table) Lead. I have tried a number of measures but none return the minimum date for each lead regardless of the Year and Month context. I would have thought that a simple Calculate with an ALL filter would do, but these do not ignore the Year and Month context in the matrix. Here is a screenshot:

Please help! I've been pulling my hair out on this for a couple of days!

Thanks,
Paul.