Postfix masquerading or changing outgoing SMTP email or mail address

Address rewriting allows changing outgoing email ID or domain name itself. This is good for hiding internal user names. For example:
SMTP user: tom-01
EMAIL ID: tom@domain.com
Server name: server01.hosting.com

However when tom-01 send an email from shell prompt or using php it looks like it was send from tom-01@server01.hosting.com

In some cases internal hosts have no valid Internet domain name, and instead use a name such as localdomain.local or something else. This can be a problem when you want to send mail over the Internet, because many mail servers reject mail addresses with invalid domain names to avoid spam.

Postfix MTA offers smtp_generic_maps parameter. You can specify lookup tables that replace local mail addresses by valid Internet addresses when mail leaves the machine via SMTP.

Open your main.cf file
# vi /etc/postfix/main.cf

Append following parameter
smtp_generic_maps = hash:/etc/postfix/generic

Save and close the file. Open /etc/postfix/generic file:
# vi /etc/postfix/generic

Make sure tom-01@server01.hosting.com change to tom@domain.com
tom-01@server01.hosting.com tom@domain.com

Save and close the file. Create or update generic postfix table:
# postmap /etc/postfix/generic

Restart postfix:
# /etc/init.d/postfix restart

When mail is sent to a remote host via SMTP this replaces tom-01@server01.hosting.com by tom@domain.com mail address. You can use this trick to replace address with your ISP address if you are connected via local SMTP.