Perhaps she signed-off her email/post/comment with a female name? You know, it is perfectly reasonable to assume gender when the name is obvious. E.g. the odds of a person called "Samantha" being male are miniscule compared to it being a female. Because in both cases, "it" is a person we are talking about.
We can't just erase gender. People think in gender, and they consequently speak it. I don't think twice about mentioning "some guy" commented on my post, for example. Just the other day I decided to be even and refer to a hypothetical person by using "s/he", and even got called out on it.
