Weird Bug: Cannot email .docx attachments

[illusionfs]

I am not able to send .docx attachments from my Mac.

OS: macOS Mojave
Clients: Mail, Outlook

If I send emails without the attachments, it works fine.
When I add a .docx attachment, the email shows as sent from the client, but is not received on the other side.

 

[xraydoc]

I am not able to send .docx attachments from my Mac.

OS: macOS Mojave
Clients: Mail, Outlook

If I send emails without the attachments, it works fine.
When I add a .docx attachment, the email shows as sent from the client, but is not received on the other side.

It is a problem sending .docx to anyone/everyone, or only one specific individual? Is it any attachment or just specifically .docx files?
Can you send them to yourself?
I'm wondering if the recipient has some filter on their end...