Evinrude just came out with a new exhaust grommet that withstands the increased water pressures on 250HO motors that have the M2 gearcase replacement. This is new compared to the "new" grommet that was included with the M2 kit.
See your dealer about getting yours replaced if needed. Your dealer can reference service bulletin 2011-04 W for parts and procedures. Evinrude is sending out letters about this, but why wait.
You can tell if you already have the new grommet by measuring the inner plastic "pee hole" tube from the hex fitting to the end. The new one is 5/8".