I use Outlook 2016 to send out invoices for service calls for our business. I've sent out hundreds of invoices on a form that I made. There's never been a problem until yesterday. A recipient advised me that the attachment was unreadable due to being chopped off on the right side. There appears to be no problem with the original document saved on my computer. But when I open the attached document from the sent items folder for that account, Outlook keeps asking if I want to merge the changes with the original document. Options are 'yes', 'no' and 'no, and don't ask again'. No matter which option is selected, the error comes back every time. If 'no' is selected, the document opens and displays normally on my computer.

I followed this problem in the sent items folder all the way back to 03/15/17. I sent an invoice attachment on 03/14 which opened without the annoying 'merge changes' prompt. However, beginning on 03/15, every attachment I sent invokes the 'merge changes' prompt when the attachment is opened from sent items.

I've only had one recipient to complain that the attachment was unreadable, but I still don't know what is would have caused the 'merge changes' question to suddenly start popping up on every attachment sent after 03/15 until yesterday. I checked Windows update history and noticed some large Windows 10 cumulative updates that were pushed through on 03/14. I suspect more than coincidence that the annoying 'merge changes' prompt suddenly started up on 03/15.

Yesterday, after researching this issue, I went into the Outlook Trust Center attachment handling settings and unchecked the box to add properties to attachments to enable reply with changes. This has stopped the 'merge changes' prompt from popping up every time, but not sure if it will lead to more problems further down the road.