Authentification issue under Debian 8.04 / sysCP

[MrTwinEngine]

Hey folks,

I really hope someone can help me out with this problem:

We have a new dedicated server with a small ISP here in Germany. The server runs Debian 8.04 and uses the newest release of sysCP as web administration interface.

So far no rocket science

I am running a couple of domains (all administered using sysCP), with a couple of email accounts.

Now, I installed the latest release of Roundcube (roundcubemail-0.2-stable.tar.gz). During the install on page three, Roundcube wants me to input credentials to check the connectivity to the IMAP/SMTP servers.

The result is always the same, no matter what account I use:

SMTP:
Trying to send email...
SMTP send: NOT OK(Authentication failure: Invalid response code received from server (Code: 535))

IMAP:
Connecting to mail.web-consult-wetterau.de...
IMAP connect: NOT OK(Authentication for [email protected] failed (AUTH): "a000 NO Authentication failed. ")

Can anyone please help me out? I'd appreciate any help.

Cheers
Niko

PS: If I omitted important information on the system, please let me know.