TODO

SASL Authentication
- Include code to fetch an OAuth2 token?
IMAP4 Client
- Consolidate MessageFlagsChanged, MessageLabelsChanged, and ModSeqChanged events into a single event?
- Extensions:
  - BINARY
  - CATENATE
  - LIST-EXTENDED (Note: partially implemented already)
  - CONVERT (Note: none of the mainstream IMAP servers seem to support this)
  - MULTISEARCH (Note: none of the mainstream IMAP servers seem to support this)
  - UNAUTHENTICATE
MessageThreader
- Fix UniqueId property to be just a UniqueId instead of Nullable<UniqueId>.
IMailFolder
- Modify Append() methods to simply return UniqueId instead of Nullable<UniqueId>?
- Modify CopyTo/MoveTo() methods to also return UniqueId instead of Nullable<UniqueId>?
Maildir
Thunderbird-style mbox folder trees?