http://www.downloadsquad.com/2007/10/23/gmail-gets-imap/:

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It appears that Google is working on integrating IMAP into Gmail. Download Squad reader eD! Thomas sent in a tip letting us know he came across the new IMAP setting while snooping around Gmail. However, it is only appearing in select few Gmail and Google Apps for domains accounts at the moment.
 
When we checked Gmail Help database this evening, Gmail had said that they support POP, but not IMAP. Then a few minutes later, that doc was gone, and a stack of IMAP related help docs were up. The new pages explain how to integrate the new mail feature. It's as easy as setting up POP, but with huge benefits.
 
Why is IMAP integration a good thing for Gmail? POP was a stepping stone, but IMAP pushes Gmails benefits over the top. With IMAP, users can now access their email via a desktop application like Outlook or Thunderbird, read emails, make changes, delete, and have the changes made across platforms. So if you now log into your Gmail account, the message which you read in Thunderbird, will now be marked accordingly. No more wasting time trying to sift through emails that had already been answered.
 
Take a peek under your Settings in Gmail, and click into your 'Forwarding and POP IMAP' tab to see if you are one of the lucky ones who got IMAP first.
 
UPDATE: Some users are reporting that if they log out and back into Gmail accounts, IMAP settings are showing up! There is hope!

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Does Gmail support all IMAP features?:

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Gmail IMAP is a fairly complete implementation of IMAP, but the following features are currently unsupported:
- \Answered and \Recent flags on messages.
- Folder subscriptions. All folders are always in the 'Subscribed' list.
- 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.

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