+ 822 => 'Internet message format (1982)',
+ 2822 => 'Internet message format (2001)',
+ 5322 => 'Internet message format (2008)',
+ 3501 => 'IMAP4rev1',
+ 2177 => 'IMAP IDLE',
+ 2683 => 'IMAP4 Implementation Recommendations',
+ # 5032 = 'WITHIN search extension for IMAP',
+ 4978 => 'IMAP COMPRESS Extension',
+ # 5182 = 'IMAP Extension for Referencing the Last SEARCH Result',
+ # 5256 => 'IMAP SORT and THREAD extensions',
+ # 5738 => 'IMAP Support for UTF-8',
+ # 8474 => 'IMAP Extension for Object Identifiers',
+
+ # 8620 => JSON Meta Application Protocol (JMAP)
+ # 8621 => JSON Meta Application Protocol (JMAP) for Mail
+ # ...
+
+ # examples/unsubscribe.milter and PublicInbox::Unsubscribe
+ 2369 => 'URLs as Meta-Syntax for Core Mail List Commands',
+ 8058 => 'Signaling One-Click Functionality for List Email Headers',
+
+ 1081 => 'Post Office Protocol – Version 3',
+ 1939 => 'Post Office Protocol – Version 3 (STD 53)',
+ 2449 => 'POP3 extension mechanism',
+ 2595 => 'STARTTLS for IMAP and POP3',
+ 2384 => 'POP URL Scheme',