Removing Microsoft 365 DKIM headers (step 1)

How to configure your system so that the eperi sEcure Platform will remove Microsoft 365 DKIM related headers.

About this task

Note: The configuration has no negative impact if there are no Microsoft 365 DKIM headers, so you can set it up as a preventive measure.

Connect to your Postfix server using your preferred SSH tool (e.g. Putty) and log in.
Check if pcre support for Postfix is installed:
```
sudo postconf -m | grep re
```
The result should look like this:
```
btree
pcre
regexp
```
If pcre is not listed, run the following command:
```
sudo apt install postfix-pcre
```
Edit the file /etc/postfix/master.cf and find the line starting with cleanup.

Underneath this line add the following:

  -o header_checks=pcre:/etc/postfix/header_checks

The lines should now look like this:

cleanup   unix  n       -       y       -       0       cleanup
  -o header_checks=pcre:/etc/postfix/header_checks

Create or edit the file /etc/postfix/header_checks and add the following lines at the end of the file:
```
/^DKIM-Signature:/  IGNORE
/^Authentication-Results:/  IGNORE
```
Reload Postfix:
```
sudo postfix reload
```

Your eperi sEcure Platform will now remove Microsoft 365 DKIM related headers if they exist.