The answer is "yes and now": it is easy to "fake" emails, though that actually is not faking at all. Please understand that the "From" and "To" addresses shown inside an email message are part of the messages content. From a technical point of view these addresses have nothing to do with from whom (from which account) that messages has been sent or to whom it is addressed. Those addresses (accounts) are specified on a completely different level.
In addition it is not up to the sending software to decide what is a valid message and what not, in terms of a message being genuine. That obviously would not make sense if you think about it. That is something that either the receiving side or the transmitting side have to make sure, here the email servers, especially the smtp servers accepting and routing messages.
And indeed this is the case: it is a question of your local smtp servers configuration if such a message is accepted for delivery or not. Most likely the message in your test case has been accepted because it originated from within your local network, or what the smtp server considers as your local network. For such messages typically other rules exist compared to routes received "from the outside" and this does make sense.
Further it is a well known fact that it is part of the responsibility of the smtp server administrators to make sure that their servers cannot be missused. The rules used for this are called "relaying rules", all current smtp server implementations offer configuration opions dealing with that issue.
In general you can say that
really "faking" an email is not that easy if you look at the details. If a receiver only looks at superficial details then indeed he can easily be fooled.
email is not at all a secure means of communication, it never has been and it never was meant to be. It is built on trust in general.
the only secure way to make sure that an email has indeed been sent by the party that is claimed as sender inside the message is by using "signed messages", by using a digital signature.