Can't send emails with client

I've been searching for a while now and after trying tons of different things found on google I'm trying the forums... You're my last hope :-)

I've set up a VPS with webmin/virtualmin. I configured my mail client (mail.app on OS X 10.8.2) and receiving email using IMAP works fine but I can't seem to send email whatever the settings I use.

My ISP blocks port 25 so I'm using 465 and I've enabled SSL but whenever I try to log on to the SMTP server it tells me my password is wrong.

Here's what I get in my mail log on the server:

Mar 12 00:22:42 vps21464 postfix/smtps/smtpd[19240]: connect from 122.202-240-81.adsl-dyn.isp.belgacom.be[81.240.202.122]
Mar 12 00:22:42 vps21464 postfix/smtps/smtpd[19240]: warning: SASL authentication failure: Password verification failed
Mar 12 00:22:42 vps21464 postfix/smtps/smtpd[19240]: warning: 122.202-240-81.adsl-dyn.isp.belgacom.be[81.240.202.122]: SASL PLAIN authentication failed: authentication failure
Mar 12 00:22:42 vps21464 postfix/smtps/smtpd[19240]: lost connection after AUTH from 122.202-240-81.adsl-dyn.isp.belgacom.be[81.240.202.122]
Mar 12 00:22:42 vps21464 postfix/smtps/smtpd[19240]: disconnect from 122.202-240-81.adsl-dyn.isp.belgacom.be[81.240.202.122]

Do you have any idea where to start looking?

Thanks a lot for any help...

Nicolas

EDIT:

As a test I tried logging in with a wrong use name/password on purpose and here's what's in the log in that case:

Mar 12 00:48:17 vps21464 postfix/smtps/smtpd[19759]: connect from 122.202-240-81.adsl-dyn.isp.belgacom.be[81.240.202.122]
Mar 12 00:48:17 vps21464 postfix/smtps/smtpd[19759]: lost connection after EHLO from 122.202-240-81.adsl-dyn.isp.belgacom.be[81.240.202.122]
Mar 12 00:48:17 vps21464 postfix/smtps/smtpd[19759]: disconnect from 122.202-240-81.adsl-dyn.isp.belgacom.be[81.240.202.122]