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How to Avoid Delivery Issues while Sending to Large Distribution Lists

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.

  Prerequisites

  • Global Admin rights in Exchange Online Admin Center.
  • Templafy Email Signature Server.

  Note

It's recommended to apply the modifications gradually to reduce the impact on the coverage of ESS.

  • If the 1st recommendation below is not resolving the issue, add on the top of it the 2nd recommendation (then the 3rd if necessary).

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 https://admin.exchange.microsoft.com/.
  2. Go to Recipients, then Mailboxes.
  3. Select all mailboxes to update this for all users. (Select each individually to update only select users)
  4. Click Set Recipient Limit.
  5. Set the Maximum Recipients to 1000and click Save.

RecipientLimit4.png

  Note

The maximum Recipient Limit in Exchange Online is currently 1000.

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.

To make this adjustment, please follow the steps below:

  • Navigate to https://admin.exchange.microsoft.com/.
  • Go to Mail flow, then Rules.
  • Find the Templafy mail flow rule, likely named: templafy email signatures.
  • Edit the conditions of rule:
    • Change the header name in the exception from X-UnsupportedFormatByTemplafy] to [X-ProcessedByTemplafy

Exclude the Large DLs 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 https://admin.exchange.microsoft.com/.
  2. Go to Mail flow, then Rules.
  3. Find the Templafy mail flow rule, likely named: templafy email signatures.
  4. Add another exception in the Except if section.
  5. Select the conditions The message] and To box contains a member of this group
  6. Add each Distribution List as needed

  Note

The recommendation is to start with the enterprise wise DLs, like all employees DL, and go gradually down with the exclusion.

ESS Email signature server tech_role
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