The destination, for now, is a gmail account. I can send mail internally (to anyone on my server) with Zimbra, I just can't get out to the rest of the world.
Where is the 'destination folder', is it on the same server or somewhere else? Are you actually saying you can't send email anywhere or just to a specific account?
Not only is it listening, but using command line (i.e. AUTH LOGIN, EHLO, MAIL FROM, RCPT TO, DATA, etc.) I can actually successfully send mail out from my server to any email account I want, on or off my server.
Being able to telnet into the server just confirms that it's listening on the specific port.
I've not gotten any 'bounce' messages yet, and it's been about four and a half days. Looking at my SMTP logs, it doesn't look like there's been any attempt at communication with it. It is completely empty except for the couple of emails I send via command line, but the log is as follows. (accounts and domains edited for privacy)
Without any information from the server log files it's impossible to give you any answer about why it's not being received or what's happened to it. If the mail can't be delivered then you should receive a 'bounce' message but that could take up-to five days for you to receive it.
There are about 3 repetitions of the above, all were sent via command line, not zimbra.
INFO 17:22:44,616 | james.smtpserver | Successfully spooled mail Mail129574084####-## from email@example.com on ##.##.#.# for [firstname.lastname@example.org]