Postfix masquerading or changing outgoing SMTP email or mail address

Address rewriting allows changing outgoing email ID or the domain name itself. Useful for hiding out internal user names, especially shell users on Linux and Unix boxes. For example:
» SMTP user/shell user: tom-01
» EMAIL ID: tom@domain.com
» Server name (FQDN): server01.hosting.com
However, when tom-01 send an email from shell prompt, cron job or using php, it looks like it was sent 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. It 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. Another valid case is where your email routed using a smarthost such as AWS SES. A smarthost is an email server in the cloud or at ISP datacenter via which we can send emails and have them forwarded on to the email recipients’ email servers.

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.

Postfix masquerading and changing outgoing SMTP email or mail address

Open your main.cf config file using a text editor such as vim command/nano command:
# vi /etc/postfix/main.cf
Append following parameter
smtp_generic_maps = hash:/etc/postfix/generic
Save and close the file when using vim. Open /etc/postfix/generic file:
# vi /etc/postfix/generic
Make sure tom-01@server01.hosting.com change to tom@domain.com as follows:
tom-01@server01.hosting.com tom@domain.com
Save and close the file. Create or update generic postfix table using the postmap command:
# postmap /etc/postfix/generic
Finally restart or reload postfix service:
# /etc/init.d/postfix restart
## OR ##
# systemctl restart postfix.service

Test change of postfix sender address from the CLI

When an email 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 or AWS SES. See how to set up/configure AWS SES with Postfix MTA for more information.

Sample config for AWS SES

Display map using the cat command or grep command/egrep command:
# cat /etc/postfix/generic
# grep $HOSTNAME /etc/postfix/generic

@ip-172-26-14-129.ec2.internal  webmaster@cyberciti.biz
@ec2-debian-10-server webmaster@cyberciti.biz

Set the following in your main.cf:
smtp_generic_maps = hash:/etc/postfix/generic
Create or update table using the postmap command:
# postmap -v /etc/postfix/generic

postmap: name_mask: all
postmap: inet_addr_local: configured 4 IPv4 addresses
postmap: inet_addr_local: configured 2 IPv6 addresses
postmap: open hash /etc/postfix/generic
postmap: Compiled against Berkeley DB: 5.3.28?
postmap: Run-time linked against Berkeley DB: 5.3.28?

Restart the Postfix service:
# systemctl restart postfix.service

Verify it

Let us send email using Linux console:
# echo 'This is a test' | mail -s 'test' vivek@nixcraft.org
Watch out for Postfix log file using the tail command:
# tail -f /var/log/mail.log

Aug 14 11:14:52 ubuntu-pi-hole postfix/pickup[13224]: 5F2DB8243B: uid=0 from=<root@ec2-debian-10-server>
Aug 14 11:14:52 ubuntu-pi-hole postfix/cleanup[13238]: 5F2DB8243B: message-id=<20200814111452.5F2DB8243B@ec2-debian-10-server>
Aug 14 11:14:52 ubuntu-pi-hole postfix/qmgr[13225]: 5F2DB8243B: from=<root@ec2-debian-10-server>, size=339, nrcpt=1 (queue active)
Aug 14 11:14:58 ubuntu-pi-hole postfix/smtp[13241]: 5F2DB8243B: to=<vivek@nixcraft.org>, relay=email-smtp.us-west-2.amazonaws.com[35.166.199.61]:587, delay=6.6, delays=0.03/0.02/4.3/2.2, dsn=2.0.0, status=sent (250 Ok 01010173ecae1595-3bd2fa2d-7693-4b0f-b573-1a499983f677-000000)
Aug 14 11:14:58 ubuntu-pi-hole postfix/qmgr[13225]: 5F2DB8243B: removed

Conclusion

You learned about the generic table specifies an address mapping that applies when mail is delivered using Postfix MTA. The syntax is:

foo@localdomain.local   foo@yourdomain.here
bar@localdomain.local   bar@another.here
@localdomain.local      webmaster@cyberciti.biz
@ubuntu-pi-hole-vm      webmaster@cyberciti.biz