tests: basic: Add tests whether system users can dispatch mails

tests/basic.nix

@@ -82,6 +82,19 @@ in
         I'm pretending to be someotheraddress@example.com and the mailserver should reject this attempt.
       ''}")
 
+      with subtest("send succeeds for system user"):
+          client.succeed("${sendMail "system" "" accounts."normal".address ''
+        Subject: Testmail2
+
+        Hello User1,
+        this is some text!
+      ''}")
+          # give the mail server some time to process the mail
+          server.wait_until_fails('${pendingPostqueue}')
+
+      with subtest("mail can be retrieved via imap"):
+          client.succeed("${recvMail "normal"} >&2")
+
       with subtest("mail sent to system-account is rejected"):
           client.fail("${sendMail "normal" "someotheraddress@example.com" accounts."system".address ''
         Subject: Mail to system-account