How to Avoid Delivery Issues while Sending to Large Distribution Lists

About this article

When an email gets sent to a Distribution List (DL) and the message gets routed outside of Exchange Online, Microsoft breaks down the message into separate emails for each recipient. Microsoft has set a limit on these recipient emails in Exchange Online Connectors which could be hit while sending emails to large DLs that go through Templafy Email signature server. Since Templafy is relying on these connectors to get the traffic routed for signature, emails could be deferred or blocked by Exchange Online, often revealing itself with an LED 451 error.

Templafy’s Product team is working closely with Microsoft to make these improvements within this use case. In the meantime, Templafy is recommending that the following modifications are made to mitigate any future occurrences of this issue.

1. Expand the Sender Limit
2. Set the Emails Already Processed by Templafy Add-Ins to Skip Email Signature Server

3. Exclude the Large Distribution Lists from Templafy Email Signature Server

Prerequisites

Expand the Sender Limit

The Sender Limit in Exchange Online limits the total number of recipients to which a user can send a single message (the total number of recipients added to the To, CC, and BCC lines of a message). By default, in the Office 365 multi-tenant service this is set to 500 for all mailboxes but this is expandable.

By maximizing the sender limit, this will allow Exchange Online to process a larger volume of emails, alleviating the deferring messages when sending to large Distribution Lists.

To update the Sender Limit, please follow the steps below:

1. Navigate to the Exchange Admin Center
2. Expand [Recipients] in the Left Hand Pane
3. Select [Mailboxes]
   
   
   
   
4. Select all mailboxes to update this for all users. (Select each individually to update only select users)
5. Click [Set Recipient Limit]
   
   
   
   
6. Set the Maximum Recipients to [1000] and click [Save]
   
   
   
   

   Note: The maximum Recipient Limit in Exchange Online is currently 1000. Please see Microsoft's Supporting Documentation regarding Customizable Recipient Limits in Office 365

Set the Emails Already Processed by Templafy Add-Ins to Skip Email Signature Server

To better optimize the email traffic and avoid the double processing, Templafy recommends excluding the emails already processed by Templafy’s VSTO and/or Web Add-In from the Email Signature Server through the Exchange Online Transport rule used to manage the traffic within Templafy. With this configuration, Templafy Email Signature Server will only process and stamp mail that has not been processed by the Templafy VSTO or Web Add-In. The Email Signature Server traffic will then consist of users in the scope of the Email Signature Server mail flow rule that are sending from their mobile devices along with users who do not have the VSTO or Web Add-In enabled.

To make this adjustment, please follow the steps below:

1. Navigate to the Exchange Admin Center
2. Navigate to the Templafy Email Signature Server mail flow rule
3. Edit the conditions of rule
4. Change the header name in the exception from [X-UnsupportedFormatByTemplafy] to [X-ProcessedByTemplafy]
   
   

Exclude the Large Distribution Lists from Templafy Email Signature Server

To mitigate this issue with complete certainty, Templafy recommends excluding large Distribution Lists (e.g. greater than 100 users) from being sent to the Email Signature Server. This will prevent emails sent to the affected Distribution Lists from receiving signatures while continuing to flow smoothly without hitting this sender limit due to the size of the Distribution List.

To exclude the large Distribution Lists, please follow the steps below:

1. Navigate to the Exchange Admin Center
2. Navigate to the Templafy Email Signature Server mail flow rule
3. Add another exception in the [Except If] section
4. Select the conditions [The message] and [To box contains person]
5. Add each Distribution List as needed
   
   

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