I just spent 2 1/2 hours with Cingular, trying to get technical help on this error.
It's fixed. So hopefully I can spare you that time/trouble, if you happen to get this error message in Xpress Mail.
I have a new Treo 650 (Cingular). It's terrific. Yesterday I activated XpressMail.....to synch with Microsoft Outlook on our Corporate Server. It worked great, right out of the box. never used a Blackberry...but it seemed to be working just like one......new emails came into the Treo....with an sound or vibrate alarm........almost in realtime.....all day......Great!
THEN...it stopped. don't remember what I may have done...or if I did anything. When I tried to get XpressMail to synch.....I got the message "Error S7300: Server....something or other"....and no Synch.

Here's what to fix.....this is on your desktop computer....there is a little conduit icon that has to be on your desktop or in your Toolbar...at the bottom right of the screen. You have to open that icon......and have the desktop computer "ReConnect"....and XpressMail will be up and running again.
it's terrific when working!
go Treo...go Cingular!