I have a couple of people at a client that often have issues receiving attachments in emails.
The email will not arrive at all. Most are running Outlook 2013 or 2016. It will not arrive on their iDevices either.
They have no junk-mail enabled in their Outlook only a spam service where the emails do not get caught either.
The same person tries to send an email without an attachment and it arrives.
This is from several different people sending to two different employees at the client.
I've heard people talk about Outlook misunderstanding the coding of the attachments but usually then the email arrives with a winmail.dat attached or similar.