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.

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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.

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

vcloudnine.de is the personal blog of Patrick Terlisten. Patrick has a strong focus on virtualization & cloud solutions, but also storage, networking, and IT infrastructure in general. He is a fan of Lean Management and agile methods, and practices continuous improvement whereever it is possible.

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Patrick Terlisten
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