The Emin7 is a ii (secondary function)... as Emin7 => A7 (the complete bridge changes are Emin7, A7, Emin7, F7) it infers Dmaj but never resolves (Dmaj could be a Chromatic Mediant, it has a common tone, is a Maj3rd away, and has chromatic resolutions, as Bb and D relate as keys).
Modulation through Chromatic Pivot Chords:
•These let you modulate to any key, no matter how distant.
•Practice making up chord progressions that use each of these modulation types.
So, I'm changing my answer to it's some form of a Chromatic pivot chord, (the G natural is common from Eb to Emin7 everything else moves up a half step chromatically). For all intents and purposes I should point out that this is essentially what NReilingh is saying above... but there is a name for it.
The last chord of the bridge is F7, coming from the Emin7 it's 3 chromatic half steps and one whole step shift, but clearly the F7 is the V of Bb. (The horn lick is F minor pentatonic scale descending an octave from F to F; and F is of course the V of Bb.)
Thanks to NReilingh for getting me to take a second (longer look at that)... I think we all missed the correct chord changes in the verse the first time around.
(well... let's see how quick I'll need to edit this again :)
Oh... I'll add the Verse changes here as well for completeness... ||:F7=>Eb7:|| then ||:Bb=>Eb:|| then the bridge (take me to the bridge! huh!), but there is two different "chord vamps?" (V7=>IV7 and I7=>V7).