Incoming messages are tiny!

If the sender of the message uses HTML for message composition, there isn't much you can do except for using the Bigger option to increase the size of the text, or changing the received message to Plain Text.

With a such a message open, at the menu bar go to View > Message and select Plain Text Alternative.

This is one of the downsides of using HTML for message composition - the sender controls the formatting.

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http://www.birdhouse.org/etc/evilmail.html

+Because it introduces accessibility problems. *When you write in plain text, the receiving mail client renders the text in whatever font the reader chooses. When you format email with HTML, the sender controls the formatting.* But this is a trap: You only think your message will render the same way to the viewer as it appears to the sender. *In reality, the receiver can end up squinting because the font looks so tiny, or vice versa. HTML is not rendered the same way from one viewing client to the next - all guarantee of accessiblity goes out the window.* This is especially problematic for visually impaired persons.+

Not with the Mail.app, and if this were possible, all received messages would be converted to Plain Text, and you might want to view HTML formatting with some received messages.

If you regularly exchange email with this person who is using HTML for message composition, and they are using Outlook, you can ask the sender to always send messages to you with Plain Text. Outlook includes an option by individual contacts in the address book to always send email as Plain Text to the chosen contact.

When a message is sent with HTML formatting, two versions of the message are sent - the HTML version, and a Plain Text version.