FWD suspicious email to myself, found my email to be 'nullnull <EMAIL>' (all caps). What is this?

Question

I'm applying for a job and their website required me to change my account password there, so I did so yesterday. Their automatic response email stated that I had changed the password, and not to reply, so I went on to other things. This morning I found an 'Undeliverable: RE:' in my inbox, stating that I had replied 'I changed it.' and that the receiving server denied the email several times. While copy/pasting the email data I went to forward the rejected email to myself, but found the autosuggested email to be 'nullnull <EMAIL>' (my email, in all caps, spelled correctly). I don't know what this means or how a response was sent from my email when I did not send it. I'm just beginning to study infosec, so any help or explanation is appreciated. Thank you.