I was surprised to read your posting, because I occasionally do need to email data that was assembled in a spreadsheet and I didn't recall any issues. So I have copied-and-pasted from a simple spreadsheet and it worked fine, presented as a Thunderbird table, which, clunky as it is, at least preserves the structure. A second attempt failed. What seemed to break it was that this second table had some merged cells, i.e. a header cell that straddles two ordinary columns. I don't think that Thunderbird's tables have any concept (let alone support!) for merged cells. Are you able to avoid using these? As to inserting before or after - I have had this same problem in many different contexts, most painfully, Word. I have learned, the hard way, to insert three blank lines, then paste the insertion on the second of those three lines. Then I have a blank line above and below the new item so I can shoehorn in any new text. In Thunderbird, I use the [https://addons.mozilla.org/en-US/thunderbird/addon/stationery/ Stationery] add-on which adds a useful HTML source editor. So if I lose my way, I can switch to HTML source view and put my cursor at an appropriate insertion point for new text.