You are using server-side injection with Signature 365, and automatic replies from your users have signatures injected by the service. You may also see this behaviour affecting users that are not configured to use the service through the mail flow rule.
As automatic replies are sent from the Exchange server, these cannot have client-side signatures applied and will always be sent for server-side injection. If you do not want this behaviour to occur, you must exclude automatic replies from server-side injection.
Some Signature 365 users will see signatures applied for users that are not configured for server-side injection. This is caused by an issue with the mail flow rule within Exchange online, and we are unable to alter this behaviour.
In order to prevent these behaviours, it is possible to create a mail flow rule on your Exchange online admin console to exclude all automatic replies.
Please note: This setting will affect ALL users and emails. It is not possible to set this on a per-user basis.
The new rule will be triggered before the existing Signature 365 transport rule, and will prevent all automatic replies from receiving a signature by marking the message as already processed.
Log on to the Exchange admin centre mailflow transport rules page and select Add a rule then Create a new rule.
Name the rule Prevent signature injection for autoreplies and set Apply this rule if to The message properties and include the message type.
Click Select one, select Automatic reply from the drop down and click Save
Under the Do the following section, set the options to Modify the message properties and set a message header.
Click the Enter text options, set the message header to X-Signature365-MessageProcessed and the value to true.
Click Next and leave the settings as default on the following screens. Review the settings and click Finish.
You will need to edit the rule priority to place the new rule before the Send to Signature 365 for signature injection rule in order for it to take effect. We recommend setting the new rule to priority 0 and the existing rule to priority 1.
Select your newly created transport rule, click Edit rule settings, set the priority to 0 and save the rule.