As others have suggested, you need to ensure that you send files in a format which is recognised on other people's computers.
While the whole point of the .odt format is that it's meant to be common across all modern word-processing programs, Microsoft detest the idea of people using non-Microsoft programs and have only belatedly allowed their own software (such as Microsoft Word) to work with .odt files. Recipients of your attachments who're using Microsoft Word 2010 (or later) should be able to open them but users of older versions of Word can't. (Confirmation here:
https://support.office.com/en-us/article/Use-Word-to-open-or-save-a-document-in-the-OpenDocument-Text-odt-format-77adca05-9543-45c7-a079-b344493c4776 )
To get around the problem, you need to save your documents into a Microsoft format. You can either do that for each document individually (by using 'Save As', instead of 'Save') or, probably better, by changing the default 'Save' option within LibreOffice. Instructions for both methods can be found here:
https://help.libreoffice.org/Common/Using_Microsoft_Office_and
(PS: I mystified as to why the problem should only recently have occurred. Perhaps you were using .doc, rather than .odt, in the past?)