Excel Old syntax @SUM will invoke [mailto:@] url translation inside Excel
(1) Either turn off URL translation in autoformatting before working on a new worksheet;
(2) OR after finishing a worksheet, removing hyperlinks before doing pasting to other applications.
Tips: For word, to apply "remove all hyperlinks" in one go, you must "Select All" first and then apply CTRL+SHIFT+F9 (area selection or table cell selection won't work with CTRL+SHIFT+F9). To toggle hyperlink code reveal, also you must "Select All" first and then apply SHIFT+F9.