Trying to alter postfix so that I can relay email to smtp.com

Error in /var/log/mailllog :-
... SASL authentication failed; cannot authenticate to server ... no mechanism available

/etc/postfix/sasl_passwd
have done:-
postmap sasl_passwd

in main.cf tried both:-
smtp_sasl_security_options = noanonymous and:-
smtp_sasl_security_options =

Postfix was installed as part of the server package. I didn't install it or any of its parts.

Setup guide from smtp.com asked to add password/username to sasl_passwd, but there was no sasl_passwd file on the server. I created it in /etc/postfix

Nothing seems to make any difference. Still the same error. Any suggestions welcome.

after hours of trial with the same error, I am delighted to see a new error in /var/log/maillog:-

"unknown mail transport error"

The only thing I changed was [] round the relayhost:-
relayhost=[path.smtp.com:2525]
exactly the same in the sasl_passwd file.

Not sure if this is progress or not.

Any suggestions welcome.

Edit:-
After further reading, think relayhost should be [path.smtp.com]:2525 and password file does not require port.

Anyway, nothing works. Still same errors.

Here you go this is for Centos

yum install cyrus-sasl-plain

For ubuntu (12.04 and probably 14.04) x64:

apt-get install libsasl2-modules