Tag Archives: outlook

Out-of-Office replies are dropped due to empty MAIL FROM

Today I had an interesting support call. A customer noticed that Out-of-Office replies were not received by recipients, even though the OoO option were enabled for internal and external recipients. Internal recipients got the OoO reply, but none of the external recipients.

cattu/ pixabay.com/ Creative Commons CC0

The Message Tracking Log is a good point to start. I quickly discovered that the Exchange server was unable to send the OoO mails. You can use the eventid FAIL to get a list of all failed messages.

Very interesting was the RecipientStatus of a failed mail.

550 Requested action not taken: mailbox unavailable  is a pretty interesting error when sending mails over a mail relay of your ISP. Especially when other mails were successfully sent over the same mail relay.

Next stop: Protcol log of the send connector

I enabled the logging on the send connector using the EAC. This option is disabled by default. Depending on the amount of mails sent over the connector, you should make sure to disable the logging after your troubleshooting session. To enable the logging, follow these steps:

  • Open the EAC and navigate to
  • Mail flow > Send connectors
  • Select the connector you want to configure, and then click Edit
  • On the General tab in the Protocol logging level section, select the Verbose option
  • When you’re finished, click Save

The protocol log can be found under %ExchangeInstallPath%TransportRoles\Logs\Hub\ProtocolLog\SmtpSend.

After enabling the logging and another test mail, the log contained the necessary details to find the root cause. This is the interesting part of the SMTP communication:

The error occured right after the exchange server issued MAIL FROM:<> . But why is the MAIL FROM empty?

RFC 2298 is the key

An Out-of-Office reply is a Delivery Status Notification message. And RFC 2298 clearly states:

The envelope sender address (i.e., SMTP MAIL FROM) of the MDN MUST be
null (<>), specifying that no Delivery Status Notification messages
or other messages indicating successful or unsuccessful delivery are
to be sent in response to an MDN.

So the empty MAIL FROM is something that a mail relay should expect. In case of my customer that mail relay seems to act different. Maybe some kind of spam protection.

Office 365 – Outlook keeps prompting for password

This is only a short blog post to  document a solution for a very annoying problem. After the automatic update of my Outlook to the latest Office 365 build (version 1809), it has started to prompting for credentials. I’m using Outlook to access a Microsoft Exchange 2016 server (on-premises), without any hybrid configuration. A pretty simple and plain Exchange 2016 on-prem deployment.

I knew, that it has to be related to Office 365, because the Outlook 2016 on my PC at the office was not affected. Only the two Office 365 deployments on my ThinkPad T480s and ThinkPad X250.

To make this long story short: ExcludeExplicitO365Endpoint  is the key! You have to add a DWORD under HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover.

DWORD: ExcludeExplicitO365Endpoint
Value = 1

Restart your computer and the annoying credentials prompts are gone.

Disable Outlook cached mode for shared mailboxes

This posting is ~2 years years old. You should keep this in mind. IT is a short living business. This information might be outdated.

When you use Microsoft Outlook in cached mode, what I always recommend, and you add additional mailboxes to your outlook profile, you will notice that the OST file will grow. Outlook will download the mailbox items (mails, calendar entries, contacts etc.), and store them in the OST file. This is the default behaviour since Microsoft Outlook 2010. If you want to disable this behaviour, you have two options:

  • Edit the registry
  • Use a group policy object (GPO)

Edit the Windows registry

The easiest way is to use a reg file. Copy this text into a file and save it as disablecachedmode.reg. Then double click the file and confirm, that you want to import the registry file.

Please note the version number after “Office”.

Outlook 201616.0
Outlook 201315.0
Outlook 201014.0

Make sure that you use the appropriate version number for your Outlook! Otherwise this setting is applied, but not working.

Group Policy

If you want to apply this setting on a bunch of clients, you should use a GPO. Before you can use a GPO, you have to install the necessary template files for Microsoft Office/ Outlook. These ADMX files are part of the “Office 2013 Administrative Template files (ADMX/ADML)” or “Office 2016 Administrative Template files (ADMX/ADML)” package.

Copy the Outlk16.admx or Outlk15.admx files to the PolicyDefinitions folder (either C:\Windows or Central Store), and the Outlk16.adml or Outlk15.adml to the corresponding language folder.

Then you can create a new GPO. The desired setting can be found under User Configuration >> Administrative Templates >> Microsoft Outlook 201x >> Outlook Options >> Delegates >> Disable shared mail folder caching. Set this to “enable” and apply the GPO.

Still using Microsoft Outlook 2010?

Please note, that the GPO template for Microsoft Outlook 2010 doesn’t contain the necessary setting that controls this functionality!