Okie, I'm just finish configure my postfix. My postfix is MTA for my MUA Squirrelmail. I'm using Linux RedHat 9.0 OS.
Somehow I'm only able to send mail but not able to receive mail.
Plus I recognize when I send mail to a mailbox to such as Bellsouth.net or yahoo.com.. I see the header of the email state as user@localhost.mydomain.com. I want to change it to such as :
user@mydomain.com, and how do i do that?
And what configuration that I need to implement in the main.cf file for my MUA Squirrelmail to be able to receive mail from the outside source? Because at the moment I only can send mail.
Any idea anyone ? 
... Appreciated.
this is an old post, but i recently found it when i had a similar problem with the "localhost".myspecificdomain.com appearing in the "from" field. i hope this helps anyone that might be looking...
i set up RH FC2 with dovecot, postfix and squirrelmail. i also setup webmin because i'm not really a unix guy and it helps me a lot.
i used the command "system-switch-mail" in a terminal to switch from sendmail to postfix for my mta.
using webmin, i went to servers -> postfix -> general options
changed "what domains to receive mail for" to my specific domain
changed "network interfaces for receiving mail" to all
click save and apply
you should automatically return to the webmin postfix config. then click "Address Rewriting and Masquerading"
change "Address masquerading" to "$mydomain" (this removes the "localhost" from the sender/"from" address)
save and apply
restart postfix by clicking "stop postfix", wait for the page to reload and click "start postfix".
hope this helps. sorry if it doesn't...
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