The problem with the Autoresponder is the email set as the Sender's email address. If you use a Hotmail account as the sender address, any subscribers who also have a Hotmail address will not receive your email.
Technically the email is not sent from firstname.lastname@example.org but from our
mailservers. The Hotmail system can tell that "masking" has occurred
when the receivers gets your email since they know the email was not
sent by them even though it says its from a Hotmail email.
You can easily fix this by using a different email. Your site comes with 5 emails, you could use one of them instead.