That error message usually means that when Firefox sent the email to your address, something was automatically sent back to it. There are two common things that can cause this: Most common is if you have an automated reply or vacation reply active on your email account. The Firefox Accounts system treats any automated response as a error message from the email provider. Usually, temporarily disabling any automated replies or setting it so that the accounts@firefox.com email address doesn't get automated replies will work. In some other cases, it could be that your email provider incorrectly returned an error message that said that the email was not sent, but it actually is. Some email providers (notably Yahoo and AOL) do this quite often. Sometimes, adding the accounts@firefox.com email address to your list of contacts or trusted senders list will help solve that. Hope this helps.