dovecot: add options to selectively enable/disable the IMAP and/or POP3 listener

modules/services/mail/dovecot.nix

@@ -10,7 +10,7 @@ let
     ''
       base_dir = /var/run/dovecot2/
 
-      protocols = imap pop3
+      protocols = ${optionalString cfg.enableImap "imap"} ${optionalString cfg.enablePop3 "pop3"}
     ''
     + (if cfg.sslServerCert!="" then
     ''
@@ -62,6 +62,16 @@ in
         description = "Whether to enable the Dovecot 2.x POP3/IMAP server.";
       };
 
+      enablePop3 = mkOption {
+        default = true;
+        description = "Start the POP3 listener (when Dovecot is enabled).";
+      };
+
+      enableImap = mkOption {
+        default = true;
+        description = "Start the IMAP listener (when Dovecot is enabled).";
+      };
+
       user = mkOption {
         default = "dovecot2";
         description = "Dovecot user name.";
@@ -146,6 +156,9 @@ in
 
     environment.systemPackages = [ pkgs.dovecot ];
 
+    assertions = [{ assertion = cfg.enablePop3 || cfg.enableImap;
+                    message = "dovecot needs at least one of the IMAP or POP3 listeners enabled";}];
+
   };
 
 }