When discussing email addresses, there are two contexts to consider: the “821/5321 address” used by the SMTP protocol for mail server commands and the “822/5322 address” seen by the recipient in the To: or From: field. The latter includes a display name that is visible to the client and combines ASCII characters with a syntax which is usually marked by an atom or quoted string. However, the syntax can become an issue when creating brand-friendly addresses. Ascii gobbledygook must be used when incorporating non-ascii names through RFC 2047, and unquoted strings could result in syntactically invalid email addresses.
Excerpt from the main article:
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