Please help.

Since installing Zimbra Desktop about 2 weeks ago it has worked fine for both sending and receiving emails.

Today - for no obvious reason - I can receive emails but I cannot send them.

On checking the account set-up - if I click on Validate & Save it gives me this message:
"service.FAILURE: system failure: SMTP connect failure"

I have not changed anything.

I have now re-set the CORRECT username and password info on all 3 accounts but it still will not send. The info was correct anyway, and did not need re-setting, but it just made sense to double check.

Now what do I do?????????