Minor one, but a lot of the mailbox addons (e.g. Postal) also use the Alt modifier for one-click mail sending. I've added the following to the initial checks in GameTooltip:HookScript so that the Pestle functions aren't triggered if the mailbox is open.

if MailFrame:IsVisible() then return end

Ah thanks for pointing it out. I'll add that fix when I get a spare moment.