https://www.iana.org/assignments/sieve-extensions/sieve-extensions.xhtml

- [ ] RFC2852 envelope-deliverby / redirect-deliverby
- [ ] RFC3461 envelope-dsn / redirect-dsn
- [x] RFC3894 copy
- [ ] RFC4790 comparator-*
- [x] RFC5173 body
- [x] RFC5183 environment
- [x] RFC5228 encoded-character / envelope / fileinto
- [x] RFC5229 variables
- [x] RFC5230 vacation
- [x] RFC5231 relational
- [x] RFC5232 imap4flags
- [x] RFC5233 subaddress
- [x] RFC5235 spamtest / spamtestplus / virustest
- [x] RFC5260 date / index
- [x] RFC5293 editheader
- [x] RFC5429 ereject / reject
- [x] RFC5435 enotify
- [x] RFC5463 ihave
- [x] RFC5490 mailbox / mboxmetadata / servermetadata
- [x] RFC5703 enclose / extracttext / foreverypart / mime / replace
- [x] RFC6131 vacation-seconds
- [ ] RFC6134 extlists
- [ ] RFC6558 convert
- [x] RFC6609 include
- [ ] RFC6785 imapsieve
- [ ] RFC7352 duplicate
- [ ] RFC8579 special-use
- [ ] RFC8580 fcc
- [ ] RFC     regex                https://tools.ietf.org/html/draft-ietf-sieve-regex-01
- [ ]         vnd.cyrus.*
- [ ]         vnd.dovecot.*
