problems with smtp in postfix

after spending hours trying to solve this problem, i am hoping that someone may be able to help…

i am running postfix on my centos 6 platform using dovecot as the pop server. i also have spam assasin running. i have not used postfix before.

i have all elements of the mailserver working ok * it delivers mail to my email adresses set up on a number of virtual domains(when sent through another mailserver)

it sends emails from my webmin control panel

the domain mapping is being done through a hash mapping table</list> 

what i want to do is to use the server (only in some circumstances) as an outgoing mailserver as i do at present through another (non linode) account

i tried to send an email through the mailserver (from thunderbird) and get the message:

> An error occurred sending mail: The mail server sent an incorrect greeting: Cannot connect to SMTP server 173.255.205.216 (173.255.205.216:25), connect timeout.

i am using a simple user login for the smtp connection connecting to the server mail.mydomain.com (all the dns appears ok)

there is nothing in the server logs to say there is anything amiss - it appears that the connection is not being made at all. thunderbird is not asking for the password as it normally does when attemping to authenticate for the first time.

just about any outgoing mailserver settings i make return the same error message.

i have tried numerous configuration settings for postfix and very few of them make any difference at all although there has been an occasion or two when the message has been different, although i wa probably expecting a rejection in those cases.

is there something obvious that i have missed, like a module i have not installed (i thought postfix was a stand alone mailserver)

i have tried everthing that i could find on the interent , all to no avail. any help would be appreciated

rs