diff --git a/util/uuid.lua b/util/uuid.lua
index 571aa4a..089b546 100644
--- a/util/uuid.lua
+++ b/util/uuid.lua
@@ -1,9 +1,34 @@
 -- source: https://gist.github.com/jrus/3197011
 local random = math.random
+
+local function is_uuid_unexisting(uuid)
+    for p, _ in minetest.get_auth_handler().iterate() do
+        local e = mail.get_storage_entry(p)
+        for _, m in ipairs(e.inbox) do
+            if m.id == uuid then return false end
+        end
+        for _, m in ipairs(e.outbox) do
+            if m.id == uuid then return false end
+        end
+        for _, m in ipairs(e.drafts) do
+            if m.id == uuid then return false end
+        end
+        for _, m in ipairs(e.trash) do
+            if m.id == uuid then return false end
+        end
+    end
+    return true
+end
+
 function mail.new_uuid()
     local template ='xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'
-    return string.gsub(template, '[xy]', function (c)
-        local v = (c == 'x') and random(0, 0xf) or random(8, 0xb)
-        return string.format('%x', v)
-    end)
-end
\ No newline at end of file
+    local candidate_uuid = ""
+    repeat
+        candidate_uuid = string.gsub(template, '[xy]',
+            function (c)
+                local v = (c == 'x') and random(0, 0xf) or random(8, 0xb)
+                return string.format('%x', v)
+            end)
+    until is_uuid_unexisting(candidate_uuid)
+    return candidate_uuid
+end