RFC5322
When we’re looking at the technical details of email addresses there are two quite different contexts we talk about. One is an “821 address” or “5321 address”. This is the email address as it’s used by the SMTP protocol, as part of the “MAIL FROM: ” or “RCPT TO: ” commands sent to the mailserver. It’s defined in RFC 821, now updated by RFC 5321, hence the name. If someone mentions the “envelope” or they’re talking about “bounce addresses”, this is the sort they mean. We’re not talking about them in this post. The other is an “822 address” or a “5322 address’. They’re the ones the recipient sees in the To: or From: headers. They’re named after their RFC, RFC 5322. This is the sort of email address most folks mean by default, unless they’re explicitly talking about the envelope of an email, but if someone describes an email
It looks like Microsoft are getting pickier about email address syntax, rejecting mail that uses illegal address formats. That might be what’s causing that “550 5.6.0 CAT.InvalidContent.Exception: DataSourceOperationException, proxyAddress: prefix not supported – ; cannot handle content of message” rejection. Why do we care? It’s good to send syntactically valid email in a warm fuzzies sort of way – it shows we know what we’re doing, and aren’t dodgy spamware – but it’s increasingly important to delivery as mailbox providers are tightening up on their syntax checks. But why are mailbox providers doing that? One reason is that authentication tech like DKIM and DMARC is built around them only being applied to email. Not to messages that kinda look like email. There are ways to bypass DKIM protections by sending invalid messages. As one example, if you send multiple copies of the From: header with different values a DKIM checker