&gt; It intercepted the not-quite standard response header &quot;Content-Encoding: ps-bzip2&quot; to our windows client application, stripped off the header it didn&#x27;t understand but, of course, didn&#x27;t decompress the payload.<p>I think this is actually a feature in many different products from different vendors. If I recall correctly, ISA Server (since 2004!) and the like inspect HTTP and SMTP traffic and validate it for conformance to published standards. If a malformed SMTP message comes in, it discards it. This prevents your mail server from being exposed to malformed messages, which could lead to denial-of-service&#x2F;remote-code-execution&#x2F;maybe it will be fine who knows.