"I am sending mom's email."
Translation:Ninatuma barua pepe ya mama.
I mean...that's the whole reason - because barua is in the N/N noun class. Nouns in the N/N noun class have no pattern anyway e.g. kalamu, nyumba, meza etc (also, 'pepe' is not an adjective, not a noun). Practice will help you learn to identify nouns and the noun classes, but some of them just don't have easily identifiable patterns