Formatting emails when format is text/plain

We are sending alerts to both email and threema(via an email) and have until now set our email format to text/htlm.
Using the templates I had beautiful emails but, unfortunately, less nice threemas.
We have now changed to text/plain to get usable threemas but the emails in text format are quite painful to read.
The formatting I had made is not being applied to the text emails.
Is there a way to create a template for text emails? Any examples out there?