FIG. 1 is a perspective view of the messenger bag. FIG. 2 is front view of the messenger bag of FIG. 1. FIG. 3 is a rear view of the messenger bag of FIG. 1. FIG. 4 is left side view of the messenger bag of FIG. 1. FIG. 5 is right side view of the messenger bag of FIG. 1. FIG. 6 is a top view of the messenger bag of FIG. 1; and, FIG. 7 is a bottom view of the messenger bag of FIG. 1. The broken lines shown on the drawings represent stitching and are part of the claimed design.