You try dealing with a million responses on every campaign and let us know how it worked out
No reply emails are a tacit admission that the emails are low-value spam.
I also like knowing when there are new versions of various assets I've gotten from the Unity asset store, these notifications come from no-reply@unity3d.com.
I am subscribed to the headlines from my local newspapers, one of them is delivered from noreply@patch.com.
My utility company sends payment confirmation emails from noreply@unitil.com.
I like knowing about the occasional blog post from authors I follow on Goodreads, these come from no-reply@mail.goodreads.com
I like knowing when games on my Steam wishlist go on sale, these notifications come from noreply@steampowered.com
I could go on...
Marketing spam is one, but far from the only, use for noreply emails.
If not, probably the million emails send in the campaign are just spam.
If you can’t afford to pay people to deal with the remaining replies, you should be better at targeting people who actually want your messages.