Change Fonts & Colors settings in Mail on Mac
Use Fonts & Colors settings 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 settings in the Mail app on your Mac, choose Mail > Settings, then click Fonts & Colors.
Message list font
Set the font and font size for viewing the list of messages.
Set the font and font size for writing and viewing messages.
If the sender used a specific font for a message, that font is used when viewing the message.
Set the font and font size for writing and viewing messages that are in plain text format.
You must select the “Use fixed-width font for plain text messages” checkbox for the font you selected to be applied.
Use fixed-width font for plain text messages
Apply the fixed-width font you selected to messages that are in plain text format.
Fixed-width font displays every character with the same width, and is useful when you’re trying to align text using spaces instead of tabs.
Color quoted text
Show quoted text in colors. To change the colors used, click the color pop-up menu for a level, then choose a different color, or choose Other for additional choices.
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 for them.