diff --git a/lib/mv_web/live/member_live/index.ex b/lib/mv_web/live/member_live/index.ex
index 1be35b4..da1b6cf 100644
--- a/lib/mv_web/live/member_live/index.ex
+++ b/lib/mv_web/live/member_live/index.ex
@@ -1644,11 +1644,13 @@ defmodule MvWeb.MemberLive.Index do
     selected_count = Enum.count(members, &MapSet.member?(selected_members, &1.id))
     any_selected? = Enum.any?(members, &MapSet.member?(selected_members, &1.id))
 
+    # RFC 6068: mailto URI params must use %20 for spaces, not + (encode_www_form uses +)
     mailto_bcc =
       if any_selected? do
         format_selected_member_emails(members, selected_members)
         |> Enum.join(", ")
         |> URI.encode_www_form()
+        |> String.replace("+", "%20")
       else
         ""
       end