When OOF makes you hit EXO limits and you’re blocked from receiving emails

I recently came across something you might also run into at this time of the year or when a company wide announcement needs to be made via e-mail and the sender is in Exchange Online:

A person sent some season’s greetings, which results into being blocked by EXO as a limit was reached (you can find more about these limits here). My first thought was something like sending e.g.: 10.000 messages per day or too many recipients, but it turned out to be something different.

Symptom

The person (needless to say someone from higher management) send to all employees an email following best practises:

  • use distribution groups
  • no mail merge or 3rd party system, which sends a dedicated messages for every recipient
  • no junks with a lot of recipients in Bcc

However, the person received shortly the following message as a vast amount of recipients had already turned on their OOF:

With this, it was clear that the person couldn’t receive any email for the next one hour as outlined:

Receiving limit: This limit applies to the number of messages per hour from any and all sources. This includes messages from internal senders, messages from the internet, and messages from on-premises servers. When the receiving limit has been exceeded on a mailbox, messages sent to the mailbox will be returned to the sender in a non-delivery report (also known as an NDR or bounce message) stating that the mailbox has exceeded the maximum delivery threshold. After an hour, the limit will refresh and the mailbox will be able to receive messages.

What has this person done wrong? Nothing!

I started my investigation and quickly found that due to automatic replies (especially OOF) from recipients the limit of 3600 messages per hour was hit.

I want to highlight that we are focusing on internal recipients!

Until this incident, it was not clear to me that internal OOF messages are also count against this limit.

Workaround

After some brainstorming and reviewing some documentations, I found the following:

Exchange has a mechanism to suppress these kind of automatic replies. It’s controlled by a MAPI attribute, which is interpreted by a MIME writer and adds a specific header with the respective value. The attribute is named PidTagAutoResponseSuppress and the documentation can be found here.

PidTagAutoResponseSuppress property valueX-Auto-Response-Suppress header valueMeaning
0x00000001DRSuppress delivery reports from transport.
0x00000002NDRSuppress non-delivery reports from transport.
0x00000004RNSuppress read notifications from receiving client.
0x00000008NRNSuppress non-read notifications from receiving client.
0x00000010OOFSuppress Out of Office (OOF) notifications.
0x00000020AutoReplySuppress auto-reply messages other than OOF notifications.
Source: Microsoft

Sadly Outlook or OWA doesn’t support this natively. I’ve found the following ways to set this attribute and avoid this issue:

Exchange Transport Rule

A very simple rule would be to set the header on any email from a specific sender:

Set the header on any message from a specific sender

As this is very static and the user won’t have control on which message the attribute is set, you can do it on specific words or combination in the subject or body:

Here the header is set, when the subject or body contains ‘[NOOOF]’. With this the user controls, when the header is set and he could do it in a way no-one realize it:

Just add the identifying symbols and change the color to match the background. With this the recipient will most likely not recognize (unless all messages are read in plaintext).

Outlook macro

This workaround is available only for Outlook on Windows. It is more complex and comes with more effort for the user and is also ONLY available on the client you have added the macro. Unless you export and import it to other client. However, this doesn’t need any Exchange admin as you set the attribute with your client.

The code for this can be found on my GitHub repository here.

Note: This was my first time I wrote some macros for Outlook! Be patient and I’m open for any code better than mine!

I added the code on my client and added the macros to my ribbon. How you add a macro to your ribbon can be found here.

Conclusion

I hope this helps some of you and you can avoid having your CEO or someone else blocked from receiving emails for an hour just because of some Season’s Greetings or important announcement to all employees.

I wrote that those are workarounds as I would like to see a solution, which enables users to control this natively with any client. Or Microsoft has another way to avoid counting such emails against this limit or avoid them.

2 thoughts on “When OOF makes you hit EXO limits and you’re blocked from receiving emails

  1. We’ve seen this before, we now use mail flow rule to simply redirect OOF messages to another mailbox for that period.
    No one in our higher management reads those OOF, plus its a hassle for EA to trash OOF in bulk without having to delete important email in between 1000’s of OOF.

    Like

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