Hi,
it seems to me that RoundCube (0.9.0) can't authenticate to an Exim SMTP server (v4.72 on Debian) using AUTH PLAIN. I'm not sure which one of them is faulty here (or is it me?), but it looks like RoundCube is probably willing to send auth information after in a separate line than the AUTH PLAIN command, meanwhile Exim expects all that in the single line (AUTH PLAIN dGVzdAB0ZXN0AHRlc3RwYXNz).
Is it a known problem?
Rimas