Problem with networking?

[gcn53 at vfemail.net]

Sorry if this is a stupid question but I'm quite new to FreeBSD.  

I have a virtual server runing FreeBSD that I mainly administer via WebMin. 
I've been trying to set up a personal email service using imap-uw, squirrelmail
and sendmail.  During testing I have been telnetting into the sendmail port.  Up
until about 2 days ago the "Mail From:" command was being accepted, now it
returns "Domain of sender address xx at yyy.com does not exist".

Also if I ssh to the box, I can run an ftp session, but can only "open" using IP
addresses, not domain names.  If I ping a domain I get " cannot resolve xyz.com:
No address associated with name".  If I ping an IP address I get "socket:
Operation not permitted".

I was investigating /stand/sysinstall a couple of days ago and I think I have
somehow messed up some network settings.  I have tried rebooting through WebMin,
but to no avail.

Can someone point me in the riht direction, please?

TIA
Graeme