Messages with large attachments do not have a signature appended

Scenario

Messages with large attachments do not have a signature appended.

Reason

When you run the connector setup wizard, a message size limit exception is added to the Office 365 transport rule - Identify messages to send to Exclaimer Cloud.

The message size exception is determined by the following:

  1. Take the MaxSendSize from the default plan, returned by Get-MailBoxPlan 
  2. Take 75% of the value 
  3. Add the result as an ExceptIfMessageSizeOver clause to the transport rule

As a result, any messages that exceed the specified limit size will not have a signature appended (because they would meet the exception on the transport rule).

Note: the maximum limit varies based on email client/type. For Exchange Online limits, please refer to this TechNet article

Solution

You can change the message size limit, as detailed in this Microsoft article. However, rather than changing the limit per mailbox (as per the focus of the Microsoft article), the change should be applied to the entire mailbox plan, using Remote PowerShell. The command required is:

Get-MailboxPlan | Set-MailboxPlan -MaxSendSize 75MB -MaxReceiveSize 75MB

This command will update the default MaxSendSize limit for the mailbox plan to 75MB, and (provided you recreate the transport rule within Exclaimer Cloud settings) Exclaimer Cloud will base the transport rule size limit on this new value.

The above command changes the default setting for new mailboxes. Exclaimer Cloud's transport rule will use the new size limit from the mailbox plan, but any existing mailboxes will continue to use their existing limits. This means that a mailbox can have a smaller (or larger) size limit than the plan default. In this case, you can apply the plan's new size limit settings to all existing mailboxes, using the command below:

Get-Mailbox | Set-Mailbox -MaxSendSize 75MB -MaxReceiveSize 75MB

Changing the limit on individual mailboxes in this way does not affect the size limit set when Exclaimer Cloud (re)builds the transport rule.

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