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Outlook on the web (earlier known as Outlook Web App) is the online version of a desktop Outlook application. If you are working remotely, it will be beneficial to log in to your Exchange account using Outlook on the web. It will retrieve your mailbox with all its folders. But, if it does not find your web address after connecting with the server, it gives a bad request error with the number 400.bad request error with the number 400

"This error (HTTP 400 Bad Request) means that Internet Explorer was able to connect to the web server, but the webpage could not be found because of a problem with the address."

When the user proxies an HTTP request from an Exchange Server 2013 Client Access Server) or Microsoft Exchange Server 2016 (running client access service) to an older version of Exchange server, then you may get a bad request error like the above.

Mostly this issue is found in Exchange Server 2016 (Enterprise & Standard Edition), Exchange Server 2013 (Enterprise & Standard Edition), and Exchange Server 2010 (Standard & Enterprise) environments.

Also, Exchange Server 2013 Client Access server show the following in the Exchange logs ( <Exchange Server Install Path>\Logging\HttpProxy\<Http resource>):

\user003,,Sid~S-1-5-21-3205615561-4199783494-2467053687-1128,Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET4.0C; .NET4.0E; .NET CLR 3.5.30729; .NET CLR 3.0.30729),,WINGTIP-E2K13,400,400,,GET,Proxy,,14.03.0123.000,IntraForest,WindowsIdentity-ServerCookie, 17:01:18,,,0,342,1,,4,14,,0,,0,,0,0,,0,46.8744,0,,,,18,0,1,0,0,1,38,1,20,20,20,24,43,,,BeginRequest=
=System.Net.WebException: The remote server returned an error: (400) Bad Request. at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult) at Microsoft.Exchange.HttpProxy.ProxyRequestHandler.<>c__DisplayClass2a.b__28();

Moreover, the Exchange Server 2010 and Exchange Server 2007’s Client Access Server may have the following error message in their HTTPERR logs:

2014-07-24 16:48:06 53335 443 HTTP/1.1 GET /owa/ 400 – RequestLength –
2014-07-24 16:48:06 53335 443 HTTP/1.1 GET /owa/ 400 – FieldLength –

Cause of the HTTP 400 Bad request error

The cause of the error is the presence of your account in the multiple Active Directory Groups. The error may occur in the proxy request form the Client Access Server of Exchange Server 2016 or 2013 to that of Exchange Server 2010.

Resolve the HTTP 400 Bad request issue

To resolve the issue, you need to remove the user account from multiple Active Directory Groups. Also, increase the MaxFieldLength and MaxRequestBytes subkeys for all Exchange Server 2010 CAS.

Method #1

First, we will learn to change the subkey parameter in the Exchange Server 2010.

  1. Go to each Exchange Server 2010 Client Access Server and try to locate the following subkey;
  2. Check the MaxFieldLength and MaxRequestBytes entries and change the value in their tables:
    Value Name Value Type Value Data Value Base
    MaxFieldLength DWORD 65536 Decimal
    MaxRequestBytes DWORD 65536 Decimal
  3. Restart the Client Access Server and try to run the proxy process again.


  • If you do not find the entries for the MaxFieldLength or MaxRequestBytes, then create these entries manually.
  • Increasing the values of these entries will allow larger HTTP packets to be sent to IIS and will cause the Http.sys to use more memory. It may further increase the chances of malicious malware and spyware attacks.
Method #2

The second method is removing the user account from the different role groups of the Active Directory. A user account can be a member of multiple groups. The Bad Request error occurs when a user account that is running the proxy service is a member of multiple groups.

You can remove the user account from the group using the Exchange Shell cmdlets. Here is the example –

Remove-ADGroupMember -Identity <RoleGroupName> -Members <AccountName>

Example –

Remove-ADGroupMember -Identity Networking -Members Jonathan

The command will remove the account name Jonathan from the Networking Role Group.

The above-mentioned methods remove the causes affecting the proxy requests and giving the bad request error 400. But such error may occur due to the corruption in Exchange mailbox too. The manual methods will not remove the corruption and the issue will keep affecting in Outlook Web Apps. So, you should also scan the EDB file for corruption issues. Kernel for Exchange Server can fix database from any kind of corruption and align the recovered data. Later, you can export data to another Live Exchange that is accessible and not showing any errors.

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HTTP 400 Bad Request error can occur when Exchange 2013/2016 Client Access Server proxies an HTTP request to an older version of Exchange Server. To fix this error, you need to remove the user account from some AD groups or change MaxFieldLength and MaxRequestBytes subkey values for Exchange 2010.

Kernel for Exchange Server