Gmail IMAP is a fairly complete implementation of IMAP, but the following features are currently unsupported:
- \Answered and \Recent flags on messages.
- Substring search. All searches are assumed to be words.
- Searching arbitrary headers. Only some headers are available for searches: From/CC/BCC/To/Subject.
- There is no SIEVE interface to Gmail filters.
- Only plain-text LOGIN over SSL tunneled connections are supported.