Try enabling both of the following options, then restart Outlook. Do not change any other settings.

• “On fast networks, connect using HTTP first, then connect using TCP/IP”
• “On slow networks, connect using HTTP first, then connect using TCP/IP”.

To get to that option:

Outlook 2010:

• File tab
• Account Settings
• Select “Microsoft Exchange” account, click “Change”
• Click “More Settings”
• Switch to the “Connections” tab
• Click “Exchange Proxy Settings…”

Outlook 2007:

• Select “Tools->Account Settings” menu command
• Select “Microsoft Exchange” account, click “Change”
• Click “More Settings”
• Switch to the “Connections” tab
• Click “Exchange Proxy Settings…”

Outlook 2003:

• Select “Tools->E-mail Accounts” menu command
• Click “Next”
• Select “Microsoft Exchange” account, click “Change”
• Click “More Settings”
• Switch to the “Connections” tab
• Click “Exchange Proxy Settings…”

Do not forget to restart Outlook for changes to take effect.

See more tips on improving Outlook performance and stability.

Do you have too many messages in your Inbox? Try SimplyFile  – Intelligent filing assistant for Microsoft Outlook.

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If you are experiencing one of the following issues, you might be affected:

• Slow performance while switching between Outlook folders.
• Missing AutoArchive functionality.
• Issues with password authentication while sending or receiving email.

Microsoft recommends uninstalling update KB2412171 if you experience any of the above symptoms. Only Outlook 2007 is affected. You can find more details in this article. Microsoft also published  KB2485531 on this issue.

Update: On January 11 2011, Microsoft issued an update which is supposed to fix the three issues listed above.

To Disable Outlook 2010 COM Add-ins

• Select “COM Add-ins” in the Manage drop-down at the bottom of the ”Options->Addins” window and click “Go”. Note: To get to the Options window, click on the File tab first.
• Uncheck any add-ins that you are not using.
• Click OK.

Do not forget to restart Outlook for changes to take effect.

See more tips on improving Outlook performance and stability.

 If you are using an older version of Outlook, see:
How to find and disable addins in Outlook 2007
How to find and disable addins in Outlook 2003

Do you have too many messages in your Inbox? Try SimplyFile  – Intelligent filing assistant for Microsoft Outlook.

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Related: 9 Simple steps for improving Outlook performance

1. Keep your PSTs at a reasonable size

With Outlook 2003 and 2007 you can create “new style” PST files which do not have the old 2GB limit. However, for best performance, we still recommend keeping your PST files relatively small and not letting them grow too much above 1GB.

Here are a few things you can do to keep PST files from growing:

1. Use multiple PSTs.
   Create multiple PST files and organize email into them by date or by project, client, etc.
2. Keep attachments outside of the PST.
   Large attachments can quickly fill up your PST and make it grow huge. You can use EZDetach to save attachments outside of Outlook and still keep links to them in email messages.
3. Save entire email messages outside of Outlook.
   Our MessageSave product can help you save entire messages outside of Outlook and create an email archival system using file folders.

2. Disable Add-ins you are not using

Running unnecessary add-ins may make Outlook slower. Check what add-ins you have installed. You probably have more add-ins installed than you think. Use the following instructions to see what addins you have installed and disable the ones you are not using.
How to disable addins in Outlook 2010
How to disable addins in Outlook 2007
How to disable addins in Outlook 2003

3. Compact your PST files

After you have split your messages into multiple PST files and removed some of the attachments, you need to compact the PST file to shrink its size. It is a good idea to compact your PST files periodically.

4. Enable Cached Exchange Mode

If you are using an Exchange-based mail account, enabling Cached Exchange Mode will likely improve Outlook responsiveness.

Note: If the network connection between your computer and the Exchange server is very fast and reliable and if you have a large Exchange mailbox, then disabling Cached Exchange Mode might actually result in better performance.

5. Compact your OST file (Exchange users only)

If you are using an Exchange account in Cached mode, compacting your OST file may help improve performance.

6. Keep an eye on your Desktop Search Indexers

Using products such as Windows Desktop Search and Google Desktop Search is a great way to be able to quickly find email messages. However, if you are not careful, these products my negatively affect Outlook’s and your computer’s performance. Be sure to configure them to only index the email messages and files which you need to be able to find quickly.

Another potential alternative is to use a light-weight search product. For example, there used to be an Outlook search product called Lookout. It worked well and was fairly light-weight on system resources. Microsoft bought the company that made it and the product is no longer officially offered. However, you might still be able to find a download somewhere on the Internet.

7. Keep your computer up to date with patches and Service Packs

Use Microsoft Windows Update service to keep up to date with Windows and Office patches and service packs.

8. Reset Outlook cache files 

If you upgraded from Outlook 2003 to Outlook 2007, some people have reported that reseting a few internal Outlook files helps with performance.

9. Disable RSS feeds in Outlook (Outlook 2007)

• Open “Tools->Options->Other->Advanced Options”
• Uncheck “Sync RSS Feeds to the Common Feed List”

Disable COM Add-ins

To disable COM Add-ins:

• Select “COM Add-ins” in the Manage drop-down at the bottom of the Trust Center window and click “Go”.
• Uncheck any add-ins that you are not using.

Note: If you see “The connected state of Office Add-ins registered in HKEY_LOCAL_MACHINE cannot be changed” see this article for help with disabling those add-ins.

Disable Exchange Client Extensions

Exchange Client Extensions are a different type of Add-ins. To disable those:

• Select “Exchange Client Extensions” in the Manage drop-down at the bottom of the Trust Center window and click “Go”.
• Uncheck any add-ins that you are not using.

Do not forget to restart Outlook for changes to take effect.

See more tips on improving Outlook performance and stability.

1. COM Add-ins

To view and disable COM Add-ins in Outlook 2003, navigate to:
“Tools->Other->Advanced Options->COM Add-ins…” and uncheck the Add-ins you do not need.

2. Add-in Manager

Navigate to “Tools->Options->Other->Advanced Options->Add-ins Manager…” and uncheck the Add-ins you do not need.

3. Registry Add-ins

In Outlook 2003, some Add-ins are not shown in the UI. You have to look for them in the registry.

1. Run regedit*.
2. Navigate to “HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\ Outlook\Addins”.
3. Look at all subkeys of Addins. Those are Add-ins you have installed. To disable a particular Add-in, set its “LoadBehavior” key to 2.

* – as always be very careful while performing registry modifications.

Do not forget to restart Outlook after making these changes.

See more tips on improving Outlook performance and stability.

To compact a PST file:

• Right click on the top-most folder of your PST. (It is usually called ”Personal Folders”, but may also have a different name).
• Select “Properties for…”
• Click “Advanced”
• Click “Compact Now”

See more tips on improving Outlook performance and stability.