SMTP Error (-1): Authentication failed

[rsbenedict]

Our clients have been receiving this error lately when they've been sending mail via RoundCube. It ALWAYS happens the first time they hit the send button, but subsequent presses of the button will get the mail sent.

When checking the logs, the error usually states that there is no such user, but  then ends up sending it when the client hits the send now button a few time.

We've had our system admin working on this for a few days and still have not been able to resolve the issue.

Any ideas?

Thanks!