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I have two mailservers configured, and the Quick Reply only lets one send mail from the default mailserver. When an attempted send failed, the textarea seems to have "grown" a few pixels to the left and the Reply popup button is now hidden and cannot be pressed. Resizing the message pane by moving the splitter between the list pane and the message pane did not resolve the issue. Unmaximizing then remaximizing the Thunderbird window did not resolve the issue. Switching to a different message then switching back did resolve the issue.
See attached screenshot.
Thunderbird 16.0.1, Conversations 2.5.2, Kubuntu Linux 12.10
It seems so.
Frustratingly, I cannot reproduce it as now all my replies are using the correct outgoing mail server. If / when I reproduce the issue I'll post another screenshot and see if I can provide any more into.