Bouncing emails

I forwarded an email to the 'guests' and 'prospects' distribution lists, and I got 15 error message emails, stating "Your message to ... could not be delivered and was returned to the FreeToastHost server."

I did the same thing last week with no errors.

What is causing this?

This is for club 670075.

Please post one of the full emails including headers so we can see what you are receiving.

I have attached one of the non-delivery returned emails that I got back.

this email failed because the email address does not exist

<<< 550-5.1.1 The email account that you tried to reach does not exist. Please try
<<< 550-5.1.1 double-checking the recipient's email address for typos or
<<< 550-5.1.1 unnecessary spaces. Learn more at
<<< 550 5.1.1 support.google.com/mail/?p=NoSuchUser l138si1840180qke.400 - gsmtp
550 5.1.1 <nttthung1996@gmail.com>... User unknown