@Marylin If the .docx file won’t open in eM Client when you associate it in Windows as @Son-of-A-Gun advised, and won’t open when saved out of eM Client either with the same corrupt error messages, then could be a few reasons.
You might have some sort of optional email antivirus scanning program running on your pc (in the background), that thought the .docx file was a possible virus when the email was received and tried to automatically repair it and failed. So the file then ended up corrupted.
So if you have any optional Antivirus email scanning programs running, try disabling that temporarily to test and then have the sender resend the original .docx file and see if you can open it again in eM Client. If that then works, you would need to disable the email attachment scanning part of any optional virus program.
Or
Something happened to the .docx file attachment in transmission prior to being received in eM Client from whichever server it passed through and then ended up corrupted when received.
So have the sender resend the original .docx file to see if its still an issue the next time received.
Or
Maybe the .docx file was created with a eg: Later version of MS Word and you need a later version of Libra Office now to open it. So make sure you have the most recent official ver of Libra Writer.
Libra Office website - https://www.libreoffice.org
Lastly check you are running the latest version of eM Client as well just incase you have an older version. Latest EMC versions from - https://www.emclient.com/release-history
Note:- I personally are using EMC V8.1.1060 and Libra Office Writer V7.0.x and have no problems opening any received attached .docx files either directly inside EMC or saved out of EMC.