Prepend X-header to all relayed messages in Postfix

I've got several production servers running a LAMP stack. They each have a local Postfix server catching any mail from the system and from PHP, and relaying it via a smarthost (the SendGrid SMTP service).

I'd like to add a custom header to every outgoing message sent to the smarthost. This allows me to filter statistics per server in SendGrid. Something like:

X-SMTPAPI: {"category": "www1"}

The Postfix docs mention using the PREPEND action in a Postfix 'access' table. So, I added the following line to /etc/postfix/access:

PREPEND X-SMTPAPI: {"category": "www1"}

and hashed the access file with postmap.

However, I have no idea how to use the map. Something like the following doesn't work:

smtp_client_restrictions = check_client_access hash:/etc/postfix/access

How do I make Postfix prepend this header?

Solution 1:

This answers your exact question: https://web.archive.org/web/20150706131729/http://hoursofop.tumblr.com/post/17760274650

Quick steps reported here:

• create a file /etc/postfix/sendgrid_headers and add this line to it:

  /^From:/ PREPEND X-SMTPAPI: {“category” : “Category Name”}
  

• update your master.cf file with the following lines:

  smtp      unix  -       -       n       -       -       smtp -o smtp_header_checks=regexp:/etc/postfix/sendgrid_header
  

It applies to a Ubuntu system and worked perfectly for me. Be careful to choose the right "smtp" line in master.cf. I used a tab to indent the -o line.

Also note that SendGrid strips out the X-SMTPAPI header from the email before it is sent on - so you won't find it there but you will see the category appear within the SendGrid dashboard.

Solution 2:

You seem to have mis-spelled header_checks as smtp_client_restrictions, which isn't even the correct spelling of the wrong parameter. ☺

This sort of thing is far better done with a simple shim around sendmail, that your PHP (or whatever) scripts are configured to use, you know. The shim script would be a simple exercise in the use of the cat and echo commands. The MTS is really the wrong place to be doing this.