there was this other, similar town pronounced -bruh or -boro that people talked about.

You were so close: Edinburgh only got its name after Edward I invaded Scotland in 1296. Before that it (which was then a larger area than the present Edinburgh) had just been called “the Burgh”, which depending on regional variation would have been pronounced either “burg” or “boro”. “Edinburgh” referred to a smaller area within the Burgh that the king’s guards would patrol more severely due to the perceived increased risk of rebellion due to higher population density.

(/joke)

Reading through their mail merge tutorial, their method looks insanely risky: putting all addresses in “to” and rembering to click another button.