Is there anything in the sentence to suggest plurality? Why would "An e-mail from home" be wrong?
Nothing describes what the form it is. This is only Duolingo's desire. The same like the meaning of " barua" or " barua pepe" which basicly means - letter /s because it has no plural form.
"kutoka" means "from" here. It is not being used as a verb in the infinitve.
Both singular and plural forms of "e-mail" are accepted by DL. You just need to be sure you don't try to make a complete sentence out of "Barua pepe kutoka nyumbani." It's a phrase, not a sentence. It would be helpful of DL not to start with an upper case letter when they're giving us a fragment of a sentence instead of a complete sentence!
It happens several times in this course. Just an oversight, I think. (Not a question, or it's a bad example of a question and the question mark is missing.)