What do you mean you can just make them up as you go along?<p>Suppose I have two users in my domain: bob@mydomain, alice@mydomain.<p>How can alice just make up a new @mydomain address which goes to alice@mydomain?  Okay, that part is simple: we can have an entire space of these generated by a rule, like gmail&#x27;s addr+whatever@gmail.com.<p>But then how does alice <i>invalidate</i> such an address that is misused?<p>I want it so that any address that is not valid generates an SMTP bounce; I don&#x27;t want an infinite space of aliases  that map to an address to all be considered valid, but a specific set, controlled by the user. When an element is removed from that set, then further attempts to send to it generate SMTP bounces.<p>Furthermore, I want it to be <i>completely</i> anonymized, just like Throttle are doing, as in:<p><pre><code>   &lt;random-chars&gt;@mydomain -&gt; alice@mydomain
</code></pre>
The text &quot;alice&quot; doesn&#x27;t appear in the plaintext anywhere, and cannot be reversed out of the local part.<p>EDIT:<p>To solve one of these problems, what we can do is assign to each user some random identifier of fixed length, from which further addresses can be generated. For instance alice@mydomain also gets &quot;xZa3f@mydomain&quot; when the account is created. To this local part, arbitrary characters can be appended: &quot;xZa3f4abPspamming.dickheads.com@mydomain&quot; such that this still routes to alice@mydomain.  Doesn&#x27;t handle the SMTP-level invalidation requirement though.