Why does attachment have unreadable content message when emailed?

Email recipients are having trouble opening documents I create with Libre.

Welcome to Ask.LibreOffice! You’ll have to edit your question and provide more information for the community to help you out. For example, do you send the files in OpenDocument format? If so, do your correspondents have LibreOffice or any other software capable of reading OpenDocument format? Are you sending the file from within LibreOffice (menu File > Send > Document as e-mail…) and using MS Outlook? What version of LibreOffice are you using? What Operating System?

It looks like your fellows don’t have LibreOffice installed. Ask them to install LibreOffice first, then they most likely can open the files you sent. If this is not the case, then please provide more information as camendil requested.