Magic Mouse won't scroll or swipe in email window, but works perfectly in Safari window.

My Magic Mouse suddenly won't scroll or swipe in the email window, top or bottom part of the window, but behaves normally in the Safari window. Replaced the Mouse batteries, 100% power, no difference. iMac is 2008, pretty old, but was updated to 10.10.3 a month ago. Bluetooth icon in upper right part of screen indicates Bluetooth is ON, and the Mouse is connected, as evidenced by proper operation in Safari. No recent upgrades/changes to email that I'm aware of.

Any suggestions?

Thanks!