TeX, if I’ve got the right app is a LaTex editor. This format (.tex) is used to generate math formulas and other difficult to layout text. It’s not supported in most email apps, and it is unlikely that Apple’s Mail app supports it. The Clipboard (Copy/Paste) only works when the content is supported in both apps.
If you need to share a non-editable version in email I recommend taking a screenshot andcropping it for details, or generating a pdf and sending that if you need to share an entire page or document.
If you need to send and editable version, then use the Attach File option in Mail. Since LaTex isn’t supported in Mail, it will just show an icon for the file.
It might be possible to copy and paste from the source code view, but the pasted result will be that code, not the result. Then again, Mail may not recognize it that way either. it all depend on how the Tex app IDs the content when it’s placed in the clipboard.