Here's a useful tip: many SMTP proxy servers don't support ESMTP. In particular, most of the SMTP proxies that clean and scan viruses don't support it. What this means to you is that if you're using a virus-scanning proxy, users aren't likely to get delivery receipts. RFC 1891 specifies how SMTP delivery status notifications (DSNs) are to be requested; if your virus scanner blocks out additional parameters to rcpt to (like, for example, rcpt to: joe@blow.com notify=failure), you won't get a DSN from that message.