Good thinking on the strainer! Now why didn't I think of that? It's always the obvious that eludes
me. Meanwhile, it seems a lot of people have problems with runny egg salad, but haven't found a solution. Omitting some of the mayonnaise is not an option, since the egg salad comes out too dry and crumbly.
Try using dry mustard instead of wet next time, although a mere 2 tsp couldn't possibly account for all that liquid. You could omit the 1 Tbsp vinegar, since mayonnaise already contains it, but then it would be a whole different recipe. In addition, I would omit the sugar - it's also already in the mayonnaise, and it creates moisture.