Speak firmly to whoever set up the email system.
They have not applied their updates. When the file goes through, it is
being recognised as a Microsoft Word file, but the extension is being
wrongly changed.
Some free-mail web services have a bad habit of doing this.
It's not a Word problem, the change is being made either by the sending
email program or the receiving email program.
Cheers
MS Word .docx files sent as attachments are received without the x, as
a .doc instead. The same is true for MS Excel .xlsx changed to .xls
This email is my business email -- Please do not email me about forum
matters unless you intend to pay!