Options > Advanced Options > Default Services.
But keep in mind, if you email someone from a different account, it will remember that account for that contact.
Say I email John Smith from
![[email address]](http://www.blackberryforums.com/?emailimage=b1eaaa77e1f7b28af35040f1be1c876b)
, when my default is
![[email address]](http://www.blackberryforums.com/?emailimage=761cf9547a422ea587d57948ba7f1f67)
, everytime I email John Smith, it will use the
![[email address]](http://www.blackberryforums.com/?emailimage=b1eaaa77e1f7b28af35040f1be1c876b)
address until I manually change it back.