Some pointless pedantry: There is no singular title "Messr", because "Messrs" as the plural of "Mr" comes from the French "Messieurs".
The French singular is "Monsieur", which is abbreviated as simply "M".
"Mr." is English for Mister "M." is French for Monsieur.
I've seen this mistake made even in letters from banks and mobile phone companies. I cringe every time I see it.