RFC 5233 / START HERE
Sieve subaddresses, before the details
Sieve is the name of an email filtering language, not an acronym. Subaddressing adds locally meaningful detail to an email address.
RFC 5233 lets a Sieve mail filter compare the user and detail portions of a locally supported address. In a chosen plus scheme, ken+sieve@example.org has user ken and detail sieve.
The idea in one picture
Read local recipient
Select user or detail
Apply a filter rule
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
This January 2008 Proposed Standard replaces RFC 3598. It helps filters sort mail by the recipient address used for delivery, while leaving separator rules and address support to the mail system. It adds no universal plus-addressing service.
A familiar example
Think of a sorting desk reading a locally agreed note on an address label. The desk must know that postal system’s format; another system may write its notes differently. A note saying admin does not prove identity or grant access.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Read the recipient that caused this deliverySubaddress comparisons are most useful when they examine the local recipient used to deliver this message.→
- Split using the mail system’s actual encodingThe separator, the order of the parts and repeated-separator handling belong to the encompassing mail system.→
- Distinguish missing, empty and compared detailsNo encoded detail is different from a separator encoding a zero-length detail.→
- Run a declared filter without inventing delivery guaranteesThe subaddress capability enables tests; separate Sieve actions decide what happens after a match.→
This introduction explains RFC 5233 (2008). Read the original RFC ↗