Added sieve rule to change mail subject

mailserver/sieve.md

@@ -1,6 +1,6 @@
 # Sieve
 
-### Sieve rule to automatically sort mails by alias
+### Rule to automatically sort mails by alias
 
 This Sieve rule filters mails by the mail alias they were sent to.
 Specifically, it uses a custom suffix separated by a dot: `.([0-9a-zA-Z]*)`
@@ -17,3 +17,23 @@ if allof (header :regex "to" "^<name>.([0-9a-zA-Z]*)@<domain.tld>$")
         stop;
 }
 ```
+
+### Rule to change Subject of a mail on server
+
+My alma mater introduced a change that subjects of all "external" mails are prefixed with `[Extern]`.
+I don't like that so I wrote a Sieve rule to remove it. The rule relies on some functions which need to be `required`.
+
+```
+require ["fileinto", "editheader", "variables", "regex"];
+
+if allof (header :contains "subject" "[Extern]")
+{
+    # Match the subject w/o "[Extern]"
+    if header :regex "Subject" "^\\[Extern\\] (.*)$" {
+        # Delete header
+        deleteheader "Subject";
+        # Add stripped header again
+        addheader :last "Subject" "${1}";
+    }
+}
+```