We also have the same issue, again all machines that have been effected so far are on the 1801 update of Office 2016 however only a subset of machines with that update are seeing the problem.
As a test we tried reinstalling ESET on a couple of the machines with issues which seemed to have resolved the problem at least temporarily, turning off the add-in as already suggested also fixes the problem.
As the users are usually unaware of the issue until the receiving party notifies them and as two of our users were notified of issue by external contacts we've now disabled the add-in via group policy. We have other AV protection in place and whilst it's not ideal to disable the add-in it's a better option than sending our external contacts empty emails given the impression that makes, also we don't have to worry about someone losing a chunk of time writing emails only to find that not only did it not make it intact to the recipient but that the copy in the sent items is also empty and they have to start from scratch.
