First, its very possible the messages you are attempting to send have been corrupted and will need to be removed from your out box.
Once this has been completed, "test" your connectivity by sending one message at a time. If your test message fails to send, Check your SMTP settings and make sure you are using the correct server and port settings.
The ability to send/receive E-mails on the web console indicates this is definitely local to the desktop client.
What is the version of the ZD you are using? Also what type of account configuration is this POP, IMAP ,exchange...
Hope this Helps
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