Recently (6/13/2017-6/21/2017) Microsoft released several updates in an attempt to address lingering security issues in Outlook 2007, 2010, 2013, and 2016. While these updates were able to correct these issues they also caused Outlook to classify certain attachments as “potentially harmful” due to special characters in the pdf filename and will remove the pdf attachment if the email recipients have the same Windows update installed.
Windows Updates KB3191898, KB3203467, and KB3191932 are responsible. Since these updates were released, two fixes were created that were pulled the next day because they caused further issues.
If an email has the special characters “( ) # - / \” in subject and/or attachment names, or if for some reason they have multiple periods “…” in either it will block attachments in emails. Removal of those special characters fixes the issue.
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