I usually set my postfix to relay via gmail SMTP to allow me for reliable external email delivery from my gmail address, with the convenience of the command-line. To send an email using mail utility I can simply use:

$ cat email.txt | mail email@example.com

This will generate an email with default X-Mailer tag, like:

X-Mailer: mail (GNU Mailutils 2.2)

To disable it, create a .mailrc configuration file in your home directory and unset xmailer option:

echo noxmailer >> ~/.mailrc