Is your Brother printer model listed by Apple as having AirPrint capability? <iOS: AirPrint Basics> If it is listed there, it should print from iPad Mail as well as Safari.
If not, I assume you are using a Brother printing App that has its own web browser program. Such an App can only be accessed from within other iPad Apps that have the "Open In" option. The only way such an App can print from mail, is if it also has its own E-Mail program.