My husband and I both have HTC Ones (mine is the M8). For some reason in the last couple of months we have both experienced this ... when trying to reply to an email message (using K9) we get an instant response of "Failed to send some messages". Send again, same result. We have to reboot the phone and the message sends immediately. I Googled the issue and see some folks have this issue with text messages but did not see email problems. Our email provider is GoDaddy. Any thoughts???