Outlook 2007 comes with improved IMAP support. As you might wonder what is new, here is the full list of improvements:

1. Outlook is now IMAP4 Revision 1 compatible (RFC 3501)
2. In previous versions, Outlook would pop up a dialog while synchronizing with an IMAP server. In 2007, Outlook uses a new chunking synchronizing strategy that allows users to work with IMAP items as they are being downloaded.
3. 2007 has better purging support. In 2003, you were only able to purge the current IMAP folder, now you can using Edit, Purge:
   • Purge current folder
   • Purge all folders for one IMAP account
   • Purge all folder in all IMAP accounts
   • Use the new Purge on Switch feature that purges items automatically when switching folders. This feature is disabled by default. You need to switch it on for each IMAP account separately via Edit, Purge, Purge Options, “Purge items when switching folders while online”. You can access the same dialog via Tools, Account Settings, Change for your IMAP account, More Settings.
4. You can now store your Sent Items in an IMAP folder. In previous versions, Outlook stored all sent items in a local PST and not on the IMAP server. When you send an email for an IMAP account the first time, Outlook will ask you whether to store the item in a folder on the server or not. You can access the setting at any point in time via Tools, Account Settings, Change for your IMAP account, More Settings, Folders. Please note that this setting will only be available after Outlook has synchronized with your IMAP account once (it needs to download the folder list first from the IMAP account)
5. IMAP accounts now use the Unicode PST format (introduced first in Outlook 2003) by default. In previous versions, IMAP accounts were limited to ANSI PSTs, which meant e.g. that your IMAP account could never be bigger than 2 GB. With providers already offering 2 GB IMAP accounts currently, e.g. as 1&1 does, it won’t be before long that larger than 2 GB IMAP accounts will be available widely. You should know though, that Outlook has no mechanism to convert an ANSI to a Unicode PST. The only way to “convert” your current IMAP ANSI PST to Unicode is to delete the file and let Outlook recreate it as Unicode PST. When you upgrade from an earlier version of Outlook, 2007 asks you whether it should do just that.
6. IMAP logging has also been improved in 2007. Outlook log files are created in “%temp%\outlook logging” and are named according to the account, activity (incoming/outgoing), as well as date and time of the first log entry. Outlook creates one log for each account per session. You can switch on logging via Tools, Options, Other, Advanced Options, “Enable logging (troubleshooting)”.
7. Outlook accounts now have a “Test Account Settings” button. You can find the button on the page where you enter the basic account information.
8. Security for IMAP accounts has been improved as well:
   • TLS encryption is now supported in addition to SSL (as required by IMAP4 Rev. 1).
   • The PLAIN authentication mechanism is now also supported. Meaning the PLAIN SASL mechanism (i.e. AUTH=PLAIN) as defined in RFC 2595 (again required by IMAP4 Rev. 1).
   • Support for STARTTLS was added (defined in RFC 2595). This allows starting SSL for an established IMAP4 session (another IMAP4 Rev. 1 requirement)
   • As for all other Internet E-mail accounts, the private ID field on SMTP outgoing messages as added. This is in reference to the new Postmark feature in 2007. More information about this feature is available on Office Online.
9. IMAP accounts (with the local PST in the Unicode format) now allow Search Folders similar to Search Folders that were already possible on regular PSTs in Outlook 2003.
10. In Outlook 2003, IMAP items could be assigned multiple different flags similar to any other email item in Outlook. This has been changed in 2007to only allow IMAP items to have a flag or no flag (flag can only be on or off). The change was made, as IMAP accounts only support such a limited flagging behavior and the different flags assigned to items in Outlook 2003 were not reflected on the IMAP server. In addition, IMAP items in Outlook 2007 cannot be assigned the new color categories, as those are not supported by the IMAP protocol either.
11. It is now easier to cache your IMAP items locally and even keep a full local copy of your IMAP account in the IMAP PST. In 2003, you had to select each folder individually that you wanted to be kept cached locally in the Send/Receive Settings dialog. In 2007, you can now choose to download all headers for all subscribed IMAP folders (meaning the ones shown to you in the folder list in Outlook) or all complete items including attachments for all subscribed folders. In addition, you can still define a custom behavior. The setting is available via Tools, Send/Receive, Send/Receive Settings, Define Send/Receive Groups, Edit for the send/receive group. Then select your IMAP account and choose the desired setting under “Receive mail items”.