https://fanisiacademy.com/courses/list-building-wizard/lesson/plain-text-emails-vs-html-emails/
Plain Text Emails vs. HTML Emails
plain text emailsvshtml
https://support.apple.com/en-nz/guide/mail/mlhlp1009/mac
Use plain or rich text in emails in Mail on Mac - Apple Support (NZ)
In Mail on your Mac, change the format of the emails you send.