Read the recipient that caused this delivery
01Subaddress comparisons are most useful when they examine the local recipient used to deliver this message.
A stopped inference prevents applying this local plus rule as a universal meaning of foreign addresses. No identity assertion or sender blocking is implied.
Source for this drawing: RFC 5233 · Read RFC section 4 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
The envelope recipient and the visible To header can tell different stories.
- Sieve is the name of an email filtering language, not an acronym. RFC 5233 adds comparisons against the user and detail portions of a subaddress: a locally supported address carrying extra routing or sorting information. In the chosen plus scheme, ken+sieve@example.org has user ken and detail sieve (§§1,4). The extension does not create accounts or require every mail provider to accept plus addresses.
- The message envelope is transport information used to deliver mail; message headers are part of the content a reader sees. For sorting based on how mail reached this user, §4 says the envelope “to” address is preferred in most cases. Mailing lists, aliases or virtual domains can leave the detail only in that envelope recipient. A visible To header need not carry the same address.
- With the separate envelope capability, Sieve’s envelope "to" test examines the RCPT address that resulted in delivery to this user. If an SMTP transaction had several recipients, only this user’s relevant most recent recipient is available, not every RCPT address or every original pre-alias destination (RFC5228 §5.4). The drawing assumes the detailed recipient is retained in this delivery context.
- The new :user and :detail selectors can be used with address and envelope tests, which act on addresses. They are not arbitrary substring selectors for a Subject header. Base address parsing, address-part restrictions and comparison rules still apply (§§4–5; RFC5228 §2.7.4). A source-route or display name is not part of the user/detail value.
- Encoding is site/implementation specific. Applying one local encoding to a foreign sender address, such as envelope "from" or originator headers, can give incorrect results (§4). A detail spelling such as admin is not an authentication or authorization credential. This guide selects local recipient comparisons; it does not interpret the null reverse-path, whose base envelope "from" matching has special empty-value rules (RFC5228 §5.4).
Fictional offline mail-filter examples. Address components are already parsed; the illustrated encoding is explicitly configured to match the local mail system. This is not a complete Sieve interpreter, address parser, provider setup or actual mail delivery. Example folders exist and illustrated actions succeed unless a failure is named.
Read the RFC section ↗