Masquerading is the process of transforming your domain name into another domain name. Then the message will appear to be coming from some other domain. There are legitimate reasons for wanting to do this. One example could be two companies that are in the process of merging. Recently an Unsolicited Commercial E-mailer was sent to prison for among other things using other domain names instead of his own. He was jailed for identity theft. Be careful how and why you do this.

Sendmail has a lot of configuration option dealing with Masquerading. Only one will be covered here. This procedure has a very narrow scope.

In some situations sendmail may append the hostname to the e-mail message. For example let's say your company is called Good Company. Your registered domain name is goodcompany.com. You have a couple of machines that receive inbound e-mail. You have a couple of machines that every internal mail server sends their e-mails to and then those machines send the mail onto the Internet. During the sending process sendmail changes user@goodcompany.com to user@some_host_machine.goodcompany.com.

You can have sendmail revert the email address back to user@goodcompany.com by using a process known as hostname hiding.

The configuration file changes needed to implement hostname hiding are:

MASQUERADE_AS(`domainname.only')
MASQUERADE_DOMAIN(`machine_name.domain_name.tld')
FEATURE(`masquerade_entire_domain')
FEATURE(`masquerade_envelope')

Since some_host_machine.goodcompany.com resides behind a corporate firewall no one from the Internet is able to connect to it to deliver the mail. So you use masquerading to change user@some_host_machine.goodcompany.com to simply user@goodcompany.com. Then when someone replies to your e-mail message their smtp server grabs goodcompany.com's mx records and sends the mail to the inbound servers who could relay relay it back to some_machine_name.goodcompany.com inside the firewall if that is the final disposition of that e-mail message. You could also configure your e-mail client's "Reply-to" option instead of doing this if you need to.

Masquerading is generally used to rewrite the sender's address. Some features let you rewrite envelope addresses and recipient headers.

With the version of Sendmail that ships with Solaris 11 I haven't found a need to change on setting.

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