Excel is a Micosoft product using Microsoft code. When you copy/paste into Thunderbird, it's impossible for Thunderbird to have all the necessary code because it is Microsoft specific, so you discover it does not look exactly the same as seen in an Excel document. It looks ok in 'Outlook' because that is owned by Microsoft. Microsoft are not going to let any other software have complete access to all it's code.

If you copy anything even normal text from a Microsoft document it bloats the email with a load of unuseable code. It's also the same if copying anything from a webpage - it copies more than just the text - it copies all the html formatting as well. This can sometimes be an issue when copy pasting images because they may include remote content. So if copying then you need to use 'Edit' > 'Paste without formatting'.

That's why email clients also have the ability to attach files such as the excel file document. This means the recipient can then use Excel on their computer to open and read the document exactly as you intended. Some people do not have Microsoft products installed but use eg: OpenOffice to read those documents. OpenOffice and LibreOffice are pretty good, but are not always a 100% accurate for same reasons - they have to interpret the microsoft code to best ability.

Another alternative as good workaround the issue: If you really need the recipient to see what you have created in Excel but they only need to read it - make the excel document full screen and take a screenshot. Then save it as a jpg image. That image jpeg file can be inserted into an email content inline or attached. REcipient will then see exactly what you see and they will not need any specific software other than any software that displays images.

Hope this info helps you.

@Toad-Hall

Thank you for taking time to explain this to me.