I have looked before, and it does not seem possible.   Must use an email client to create anything other than HTML.

Hello Douglas,

Thanks for the confirmation. I was asking myself if I missed the option.

It's not that nowadays plain text is widely used but some mailing list operator doesn't like html mails being sent to their list (especially network operators mailing lists/groups that stay oldschool... and want to reduce the size of the exchanged e-mails through their list).

Kind regards