Cannot send mail but no bounce back

[lindsaypeddle]

I have set up my Zimbra desktop but when I send out mail from two of my mail accounts they don't work. However nothing bounces back so it just seems they disappear and are never received, I know as I have tried sending to my own g-mail account and nothing arrives yet no notice of that in my Zimbra. My g-mail and yahoo accounts work perfectly both send and receive the the email I am having issues sending with is a hostgator hosted email address, though set up works perfectly as does receiving. Could it be an improper port?