In Shakespeare's Romeo and Juliet, what does Mercurio think is in Tybalt's letter to Romeo?

Tybalt sends a letter to Romeo at Lord Montague's home on the day after the young Montague and his cousins went, uninvited, to the masquerade party at Lord Capulet's.  When Benvolio tells Mercutio about this letter, Mercutio says that he believes it to contain "A challenge, on my life" (2.4.9).  In other words, he's quite certain that Tybalt's letter to Romeo contains a challenge to a fight because he anticipates that Tybalt is angry about the presence of Romeo and his friends at the Capulet party. 

Mercutio is, of course, right.  The letter does contain a challenge, or so we can assume, because Tybalt comes looking for Romeo later on, clearly looking for a fight.  In Act 3, scene 1, Romeo refuses to fight Tybalt because he just married Tybalt's cousin, Juliet, in secret.  Mercutio interprets this as a "dishonorable, vile submission," and he fights Tybalt instead (3.1.74).  Tybalt slays Mercutio, then Romeo kills Tybalt, and this is what leads to Romeo's exile from Verona.