Yea if a ticket/email change/pin change/anything that involves TNT sending an email anywhere and it doesnt exist that will bounce back to the neomail of that account.
It usually happens when the email you input is invalid. go to the email preferences on your neopets account and redo your email. if it happens again i would submit a ticket.
I got this message before on an old account of mine (Which I had the email to). I think I had to switch email addresses to fix the problem, however I'm not sure since it was quite awhile ago.