Added mail server stuff

mailserver/README.md

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+# Mail server
+
+* [Sieve](./sieve.md)

mailserver/sieve.md

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+# Sieve
+
+### Sieve 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: `.(.*)`
+It automatically creates a directory for that alias and moves the mail there.
+Then it stops processing.
+
+```
+require ["fileinto","regex","variables"];
+
+if allof (header :regex "to" "^<name>.(.*)@<domain.tld>$")
+{
+        set :lower :upperfirst "target" "${1}";
+        fileinto "${target}";
+        stop;
+}
+```