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Complete Guide When Your Exchange Server Goes Unavailable

Encountering the ‘Your Exchange Server is unavailable’ error that may occur due to network connectivity issue, incorrectly configured Outlook, missing default gateway configuration in Outlook (the client application), damaged Outlook OST file or others while using Outlook client configured with Exchange server is one of the worst-case scenarios

This error stops all internal and external communication. Consequently, impacts businesses with data and revenue loss. Considering this, get a complete guide on causes of the error and methods to fix it, resume business communication, regain the lost data, and make up for the incurred loss that happened after Exchange Server turned unavailable. 

Solutions for ‘Your Exchange Server is unavailable’ error

 Try out these solutions and check if the ‘Exchange Server is Unavailable’ error gets fixed.

Unlock Microsoft Outlook File

OST file is created when Outlook is used in conjunction with Exchange Server. At times, this OST file may get locked or set to read-only (deliberately or mistakenly), thereby resulting in the error. Therefore, check the property of OST file to verify. If you find that the OST file is set to read-only, unlock it.

How to check and unlock OST file?

Execute the steps below:

  1. On your Windows system, navigate to C: Users <username> AppData > Local > Microsoft > Outlook
  2. Right-click the OST file
  3. Click Properties tab to display ‘Properties’ window
  4. In the General tab of the Properties window, check if the ‘Read-only’ attribute is checked or unchecked. If checked, uncheck it to unlock OST file
  5. Edit Registry Values

Incorrect Registry settings may be another reason for the ‘Exchange Server is Unavailable’ error. Try to correct it by modifying the settings in the Registry. However, before editing, ensure that you have a backup to restore the original registry in case a disaster occurs.

How to edit the registry values?

Follow the steps below:

  1. On your Windows system, press Windows + R key to open the Run window
  2. In the Run window, type in ‘Regedit’ and click OK
  3. In the ‘Registry Editor’ window that appears, move to the Subkey.


Note – The Subkey are as follows:

HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\RPC for Outlook 2007, HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\RPC for Outlook 2010, and HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\RPC for Outlook 2016.


  1. In the Registry Editor, click Edit tab, hover mouse to New tab, then click DWORD
  2. Type ‘DefConnectOpts’ and press Enter key
  3. Right-click ‘DefConnectOpts’ and from the multiple tabs that come up, click Modify…tab
  4. In the Edit DWORD Value box that displays, enter ‘Value data’ as 0 and click OK button
  5. Exit Registry Editor and restart your system to ensure successful modification of Registry value

Reset Cache

Old data in the Cache may also be the cause of ‘Exchange Server unavailable’ error. In this case, run flushdns command to overcome the issue as the command resets the DNS cache server to remove or reset the cache.

How to reset Cache?

The steps are as follows:

  1. On your system, open Command Prompt desktop app
  2. Type ‘ipconfig/flushdns’ and press Enter key

‘Successfully flushed the DNS Resolver Cache’ message comes up in the Command prompt window.

Now that you have successfully flushed the DNS Resolver cache, close the Command Prompt window and try connecting Exchange Server with Outlook to see if the error gets fixed.

Disable Add-ins/Third-party apps

Sometimes, add-ins or third-party applications including anti-virus, interfere with Outlook connected to the Exchange Server and thereby the ‘Exchange Server is unavailable’ error occurs.

To check if interference with third-party applications or add-ins is the cause, start Outlook in Safe Mode which opens Outlook without the loaded add-ins or other third-party applications installed on the system that can tell you if any of the add-ins is the cause.

How to start Outlook in Safe Mode?

Execute the following steps:

  1. Open ‘Run’ box. Either click Start > Run or type Windows and R key simultaneously
  2. In the Run box, type ‘Outlook /safe’, and click OK
  3. In the Choose Profiles dialog box, accept the default setting of Outlook and click OK
  4. If there is a password for Outlook, enter the ‘Outlook password’ and select Accept when prompted

Note – If there is a label of Safe Mode in Outlook screen, it means Outlook is working in Safe Mode.


Now, if Outlook connects to Exchange Server in Safe Mode, it signifies that third-party apps/add-ins are the cause for the error and you need to remove those. Here, the strategy is to disable all add-ins at once and then enable those one-by-one to identify the add-in causing the issue. Finally, disable the identified add-in.


How to disable add-ins in Outlook?

The steps are as follows:

  1. Open Outlook and click File > Options > Add-ins
  2. At the end of the View and manage Office Add-ins, check that the Manage drop-down displays COM Add-ins. Then, select Go button
  3. When the COM Add-ins listing of the current add-ins open, clear all selected checkboxes for the available add-ins, and then click OK to disable all the add-ins

Note – Do not select an add-in and select Remove, instead disable those via the process explained above. However, if you have administrator rights, you can delete the add-ins.

  1. Select File > Exit to close Outlook
  2. Convert OST to PST file

It is possible for Outlook OST file to become damaged or corrupt, and thereby you encountered the ‘Exchange Server is unavailable’ error. In such a situation, you can convert OST to PST format, and then use the PST file in Outlook to establish email commutation and access all the stored data.

Reason for converting OST to PST file – You can import and access PST file in Outlook. However, it is not possible to access independent OST file.

How to convert OST to PST?

There are many OST to PST file conversion tools that can convert OST to PST file. However, all do not support damaged or corrupt OST file, a few, including Stellar Converter for OST software that converts OST to PST file in few steps: Select, Convert, Preview, and Save. Secured conversion, no data loss, intelligent filters, storage location/destination options, and interactive user-interface are other useful traits of the software.

Find & resolve client connectivity issue between Outlook and Exchange

As one cause of this error is the disruption in network connectivity, find and resolve the issue between Outlook and the Exchange Server by using the client version of Microsoft Connectivity Analyzer Tool.

How to use Microsoft’s Connectivity Analyzer Tool?

Execute the following steps to do so:

  1. Visit Microsoft’s site to download and install Microsoft Connectivity Analyzer Tool
  2. Click the Client button and then the Install Now button
  3. After installing, select ‘I can’t log on with Office Outlook’
  4. Enter the Simple Mail Transfer Protocol (SMTP) address and login credentials when prompted
  5. View and save the results
  6. Create a new Outlook profile

DO this if the issue occurs for one Exchange mailbox and if you do not face problem in running the Remote Connectivity Analyzer Tool.

  1. If creating a new Outlook profile does not fix the error, or if several users are affected, contact Microsoft Online Service Support and provide detailed information:
  • The time from when the issue started occurring.
  • The output of ipconfig /all, ping, and tracert command.
  • The output of telnet command output on Port Nos. 135 and 443.
  • The issue occurs in a particular ‘office network segment’ or on a particular ‘Exchange mailbox server’.
  • The output of Microsoft’s Remote Connectivity Analyzer Tool.


The ‘Your Exchange Server is unavailable’ error has substantial business implications and thereby needs to be fixed immediately on occurrence. As discussed, employ the solutions such as unlocking OST file, editing Registry values, resetting cache, disabling add-ins, removing challenging third-party apps, converting OST to PST, etc. to resolve the issue. You will get rid of the problem.

Originally posted 2018-11-06 17:24:06. Republished by Blog Post Promoter

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