No because relays can be closed, and usually are. Your ISP's SMTP server requires authentication, using the credentials they gave you, and almost certainly uses TLS also.
Your me@isp.net address (or perhaps the me part of it) is used for authenticating, along with the password. It will likely be used as your envelope address when sending; the SMTP command will be MAIL from: me@isp.net, though there could be flexibility there to accept other sending envelope identities.
In any case, your mail's From: header will have the me@hn.com.
If the ISP were to filter on the From: addresses after receiving the content of the e-mail, you'd have to negotiate something with them.
Your me@isp.net address (or perhaps the me part of it) is used for authenticating, along with the password. It will likely be used as your envelope address when sending; the SMTP command will be MAIL from: me@isp.net, though there could be flexibility there to accept other sending envelope identities.
In any case, your mail's From: header will have the me@hn.com.
If the ISP were to filter on the From: addresses after receiving the content of the e-mail, you'd have to negotiate something with them.