Q: why won´t my mouse scroll in mail? works normally elsewhere.

Hi. What is the version of OS X on your machine? Have you tried to restart/relaunch Mail application?

Unfortunately you have not completed your profile so we have no idea what OS you are using or what version of mouse you have.  Please find a minute to complete your profile, it does make it easier for us.

If you are on El Capitan and are using an old mouse, that could contribute.  Similarly, if you have bought a new mouse and not upgraded to El Capitan, that would certainly explain it.

Yes, but is it a new Magic Mouse 2.  (say, bought in the last few months)?

It sounds like an older Magic Mouse 1 version.   You cannot replace the battery in the new version, it has a direct charging cable.  

It is a known problem though infrequent.   Try going to Systems Pref > Accessability > mouse > mouse options > and I think you can check a box to scroll with inertia.

I don't have that OS and the options have changed in mine.

I know there is an inertia control in the trackpad option; just wondering if that does both jobs.   Otherwise I've run out of ideas.