Why does Facebook say my email is incorrect when I try to message someone I'm not friends with on Facebook?

[Penelope]

I was on a Facebook group and someone asked a question about something and no one knew the answer to but I know the answer but I don't want to join the group because it some freshman college thing. Anyways, I tried messaging the girl but when I go to send it it says "One or more of your email addresses are not valid. Please try again."
I don't want to email her, just message her. I don't know her or have her as a friend but I thought I'd be nice and give her the answer. But how to I let Facebook let me message her?

[abraXus]

you are misreading the message

it means "one of the emails of the person you are trying to send a message to isnt valid"

it's not saying YOUR email is invalid

and messages are the same thing as emails on facebook

she probably has her account set up to prevent people who arent her friends from sending her a message, and there is no way around that