There are two from headers.
From: - this is what you are using and most email clients will display this when you read a message with this header.
from: - this is the header that the mail program sets (sendmail) this header usually can't be changed since it would allow spamming and spoofing of addresses.
Your autoresponder might be reading the from: header, and if this is the case then it would be adding a bunch of users called www@mysite.com or nobody@mysite.com depending on what user runs your web server.