I am having the same problem as https://bugzilla.mozilla.org/show_bug.cgi?id=1459780 https://bugzilla.mozilla.org/show_bug.cgi?id=1479607 I had accidentally deleted the email address I use from my email server before trying to sign back in to Firefox Sync. I added it back, and made sure that it works. However, it is impossible for me to sign in at all any more because I'm getting the "Your Email was just Returned. Mistyped Email?" error. Please remove my email address from the bounce list so that I can sign in.