MS Word is designed to be able to perform word processing that offers way more than simple email html. If you use copy paste into an email then you will have a massive load of formatting that is included and this bloats an email with unecessary code.

If you copy paste the document into a 'Write' message, then depending upon what formatting choices you used in the document, not all formatting can be rendered and so you may see some variants in the formatting. Remember, your recipients can choose to view in Plain text (which removes all formatting) or choose what font they prefer and so alter what you originally designed in an entirely different product. They may also have a different OS and not have some of the formatting you specified anyway.

Hence why it is better to attach the msword document to an email. Then it can be read using the correct software and so seen as you intended. The document can also be detached from the email and saved separately, thus reducing the folder size and offering ability to read without Thunderbird etc etc. It appears more professional.

I've received emails from people who use Outlook and they copy paste from MS Word as well. To them the email looks fine before sending, but recipients not using Outlook, may not see exactly the same formatting. As you are now discovering, just because an email looks ok in Outlook, it does not mean others will see the same.

When I use 'OpenOffice' Writer, type document and use Send > Document as email:

Write new message opens with the document attached. I have to then choose who to send to, enter appropriate Subject and type the email, then send.

So a new Write email is opened, but the document is attached not converted into the content text of an email.