The font blizzard uses for the mail creates a nice aesthetic. Sometimes, though, you just want to read it and hurry back to separating mobs from their hit-points.
Best Usage: While reading or sending mail, type /LM and adjust till you're happy.
Nota Bene:
/lm -- loads the configuration UI
Font selections depending on the mods you have installed that also use libSharedMedia.
Settings are currently global so you only need to do it once.
For Advanced Users with their own code base, you can just use the below, using an in-game fontObject (defined in FrameXML/Fonts.xml), your own custom font, or one from libSharedMedia.