I am unable to send any e-mail to @nuspire.com This problem has nothing directly to do with eM Client, and the e-mail host is working on that problem. However since attempting to send that e-mail, it continues to fail to send. i deleted it from outbox and restarted eM Client. Still comes up error sending, but Outbox remains empty.
You could login through the web interface for your account, and access the Outbox that way.
Another option is that the message is in the Local Folders Outbox, and that is not visible. You can access it as a Smart Folder by right-click on a smart folder and choosing Display > Outbox. Or you can enable Local Folders in Menu > Tools > Settings > General > Show Local Folders.