User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:17.0) Gecko/20100101 Firefox/17.0 Build ID: 20121129050502 Steps to reproduce: I sent a message to an invalid email address Actual results: The mail server(qmail) rejected the message at smtp level with a 5.1.1 error : sorry no mailbox here by that name. I pressed ok in the error dialog window, the email composition window closed and the email was copied to sent folder. I downgraded to version 10 that behave properly. Composition window did not close and the message was not copied into sent folder This is a serious bug because users may think that the message was sent, which is not true.