Facebook email pretends disposable email is "incorrect"

I wanted to try out the email-out feature of Facebook, but didn't want to send messages to "myself" or register a new account somewhere, so just made a disposable one at guerrillamail... When I try to send it, FB replies - "please enter at least one recipient". What the....? When I add a known email and a disposable one as well, then also kicks it back: "at least one of the emails are incorrect".

Checked if things change if I add the guerrillamail, or mailinator address as my contact - that has been refused as well.

While I can see the desire by some admins to block sign-up from such addresses, is there any legitimate reason to block outgoing mail too?