Unable to set postfix as smarthost with plain authentication on port 25 (no tls/ssl): error 550 5.1.0 xxxxx authentication failed

[larzeni]

Hi there,
I'm using Debian GNU Linux "squeeze",
postfix 2.7.1-1+squeeze1

I'm in need of using a smarthost to relay all of my mail.

I'm unable to use an italia provider (aruba) as smarthos for my server.
I obtain the (in)famous "550 5.1.0 XXXXX authentication failed"

relevant part of logs:

Feb 6 13:42:42 myserver postfix/smtp[12173]: > smtp.provider.com[smtp.provider.com]:25: MAIL FROM:<ro...@myserver.com>
Feb 6 13:42:42 myserver postfix/smtp[12173]: < smtp.provider.com[smtp.provider.com]:25: 550 5.1.0 x0ih1k00U1GKSXt010ihSY authentication failed

Relevant part of configuration:

relayhost = smtp.provider.com
smtp_cname_overrides_servername=no
smtp_sasl_auth_enable = yes
#smtp_sasl_security_options = noanonymous
smtp_sasl_security_options =
smtp_sasl_password_maps = hash:/etc/postfix/sasl/saslpasswd
smtp_always_send_ehlo = yes

I've tested username/password using thunderbird as client, it works.
I've tested the same configuration with another provider: it works.

My guess is that the provider uses different server to answer to my request, and so postfix is unable to find a matching password in file /etc/postfix/sasl/saslpasswd.

But I've tried by using smtp_cname_overrides_servername=yes or smtp_cname_overrides_servername=no and it failed in the same way.

I've also tried to declare all hostnames that I can see in the logs placing all of them in the /etc/postfix/sasl/saslpasswd but even this way I cannot send my mail.

Is there anyone that can help me?
Thanks, larzeni