dovecot: add support store mailbox names on disk using UTF-8

default.nix

@@ -574,6 +574,14 @@ in
       '';
     };
 
+    useUTF8FolderNames = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Store mailbox names on disk using UTF-8 instead of modified UTF-7 (mUTF-7).
+      '';
+    };
+
     hierarchySeparator = mkOption {
       type = types.str;
       default = ".";

mail-server/dovecot.nix

@@ -29,10 +29,11 @@ let
   bool2int = x: if x then "1" else "0";
 
   maildirLayoutAppendix = lib.optionalString cfg.useFsLayout ":LAYOUT=fs";
+  maildirUTF8FolderNames = lib.optionalString cfg.useUTF8FolderNames ":UTF-8";
 
   # maildir in format "/${domain}/${user}"
   dovecotMaildir =
-    "maildir:${cfg.mailDirectory}/%d/%n${maildirLayoutAppendix}"
+    "maildir:${cfg.mailDirectory}/%d/%n${maildirLayoutAppendix}${maildirUTF8FolderNames}"
     + (lib.optionalString (cfg.indexDir != null)
        ":INDEX=${cfg.indexDir}/%d/%n"
       );