I have HP pc running windows 7 home premium

This is were everything started:
I have a free yahoo account and i was trying to set up outlook and i found out that they are not compatable. so i started to do some research and I found Zimbra desktop.

I deleted outlook and downloaded zimbra desktop and everything looks good, it synchronized all folders with no problem.

Here is my problem:
If i sent and email straight from my yahoo account to itself and try to view it on Zimbra desktop, i have no problem.
but, If i send it from Zimbra. It shows that it sent but, it never shows on zimbra inbox. why & how can i fix this?

Also have the onther problem for example, If I'm looking at Craiglist and i try to send and email to an add. I get the following message: "could not perform this operation because the default mail client is not properly installed".
how can i fix this?

I'm a computer newbie so please respond accordingly.