Use plain or rich text in emails in Mail on Mac
You can choose to send email messages in plain text or rich text (HTML) format.
In the Mail app on your Mac, do one of the following in your message:
Choose Format > Make Plain Text (available only when the current format is rich text). Plain text format doesn’t include formatting (such as bold or italic text) or items like tables and images, but is readable for all recipients.
Choose Format > Make Rich Text (available only when the current format is plain text). Rich text (HTML) format can include formatting, tables, and images, but may be unreadable for some recipients.
To use the same format for all of your messages, choose Mail > Settings, click Composing, then choose a message format. If you want to use a different format for replies, select “Use the same message format as the original message.”