Is it essential that the table comes from Excel? Because you can insert a native html table within Thunderbird, and being html it's less likely to have the formatting messed up when displayed / edited on a different machine.

Tks for your feedback. Sometimes, we just want to grab some data that in Excel and put the content into mail. We don't want to rebuild a table and insert data, that will waste some more time to do 2nd data establishment.

I don't use Microsoft software so I don't have Excel. I use LibreOffice instead which creates and edits files in Excel compatible format. I have just tried creating a new message in Thunderbird and copying-and-pasting into it a block of cells and data from an .xls file. What I found is: 'paste without formatting' removes the cell boundaries and the column alignment from the data; 'paste as quotation' retains the alignment but the cell boundaries are invisible; 'paste' retains the alignment and the cell boundaries are visible, and the content of cells can be edited. So it looks to me as if your problem is with Excel, which for some reason won't let you copy cell boundaries to the clipboard; the paste function in Thunderbird appears to work fine.