All emails that are sent via Mail Merge will have an indication of whether they've been sent via mail merge. There are two options for how you can work with this:
1) If you set up a checkbox field called "YeswareIsMailMerge__c" on your Task object in Salesforce, we can check and uncheck that appropriately for all Message Sent: tasks that Yesware creates
2) The easier way to set this up - take no action! We're including "Mail Merge: Yes" in the "Comments" section of the task created in Salesforce for any message sent via Mail Merge.