diff --git a/nixos/modules/services/mail/roundcube.nix b/nixos/modules/services/mail/roundcube.nix
index ed1439745ac9..f90f27afb828 100644
--- a/nixos/modules/services/mail/roundcube.nix
+++ b/nixos/modules/services/mail/roundcube.nix
@@ -95,6 +95,16 @@ in
       '';
     };
 
+    maxAttachmentSize = mkOption {
+      type = types.int;
+      default = 18;
+      description = ''
+        The maximum attachment size in MB. Note: Since roundcube only uses 70% of max upload values configured in php
+        30% is added to <xref linkend="maxAttachmentSize"/>.
+      '';
+      apply = configuredMaxAttachmentSize: "${toString (configuredMaxAttachmentSize * 1.3)}M";
+    };
+
     extraConfig = mkOption {
       type = types.lines;
       default = "";
@@ -115,7 +125,7 @@ in
       $config = array();
       $config['db_dsnw'] = 'pgsql://${cfg.database.username}${lib.optionalString (!localDB) ":' . $password . '"}@${if localDB then "unix(/run/postgresql)" else cfg.database.host}/${cfg.database.dbname}';
       $config['log_driver'] = 'syslog';
-      $config['max_message_size'] = '25M';
+      $config['max_message_size'] =  '${cfg.maxAttachmentSize}';
       $config['plugins'] = [${concatMapStringsSep "," (p: "'${p}'") cfg.plugins}];
       $config['des_key'] = file_get_contents('/var/lib/roundcube/des_key');
       $config['mime_types'] = '${pkgs.nginx}/conf/mime.types';
@@ -172,8 +182,8 @@ in
       phpOptions = ''
         error_log = 'stderr'
         log_errors = on
-        post_max_size = 25M
-        upload_max_filesize = 25M
+        post_max_size = ${cfg.maxAttachmentSize}
+        upload_max_filesize = ${cfg.maxAttachmentSize}
       '';
       settings = mapAttrs (name: mkDefault) {
         "listen.owner" = "nginx";