RFC 2221 IMAP4 Login Referrals
RFC 2298 Details return receipts
RFC 2342 The IMAP4 NAMESPACE command
+RFC 2369 The Use of URLs as Meta-Syntax for Core Mail List Commands and their
+ Transport through Message Header Fields
+ (implemented in listcommands plugin)
RFC 2616 HTTP/1.1
RFC 2683 IMAP4 Implementation Recommendations
RFC 2822 Format of an email message (sendmail)