Sprout Email is designed to be extensible by other plugins. One reason we have kept it separate from Sprout Forms is so that users could have the option to manage all of their email in one place, regardless of which plugins they need to manage email for.
Plugin developers can extend their plugins to support Sprout Email, additionally, you can write your own custom plugin to add support for another plugin too. For example, Sprout Email extends itself in this way to add support for several Craft Events. (You can see these examples in the
It looks like FormBuilder2 has an
onBeforeSave Event that you could write a Sprout Email Custom Event to support.
Depending on your coding skills or preferences, that might not be ideal, so it may be worth petitioning FormBuilder2 to add support for Sprout Email itself or add an
onSave event that would ensure your Form Entry was saved before triggering any email. This could at least give you complete control over your email in the short run until FormBuilder2 added additional support for what you need down the road, or it could give you a way to manage your email notification needs all in one place.