problems sending mail with atmail

[RooMeat]

I changed from Horde to Atmail a while ago as I could not get Horde running at all. Never heard from any customers so assumed it was working fine (I don't use webmail).
But.. I was informed today that it is not possible to send emails from the atmail web client.

I tried.. and sure enough.. no go.
I got the following message:

Could not send message to SMTP server. Check you have access to send messages via the server and that all To/CC/BCC addresses are valid\nError: SMTP Server rejected email. Returned:530 Authentication required

Have searched the net with no success.. so hoping someone here can help me out.