Change Fonts & Colours preferences in Mail on Mac
Use Fonts & Colours preferences in Mail to change fonts and font sizes for writing and viewing messages in Mail, and for distinguishing quoted text when replying to or forwarding messages.
To change these preferences in the Mail app on your Mac, choose Mail > Preferences, then click Fonts & Colours.
Message list font
The font and font size for viewing the list of messages in classic layout only (where the message list is above the messages). Click Select to use a different font.
The font and font size for writing and viewing messages. Click Select to use a different font.
If the sender used a specific font for a message, that font is used when viewing the message.
The font and font size for writing and viewing plain text messages. Click Select to use a different font.
You must select the option to use fixed-width font for plain text messages for the font to be applied.
Use fixed-width font for plain text messages
Fixed-width fonts display every character with the same width. This font is useful when you’re trying to align text using spaces instead of tabs.
Colour quoted text
Show quoted text in colours. Click the colour pop-up menu for a level to use a different colour.
Quoted text is the part of a message included when you’re replying or forwarding the message. It’s typically indented.
If your recipients don’t have the same fonts that you used in your message, similar fonts are substituted.