The new font is way too small and makes reading my inbox a pain. If we can't change the default font, I'm changing email providers.
If Yahoo doesn't add an option to change the default font settings of the inbox, I'm going to change email providers because the new font is way too small and completely destroys the usability of a website where 99% of the actions are reading.

This feature is unavailable in Yahoo Mail, but the text size can only be changed from your browser settings.
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Daniela Di Luzio commented
Please update your fonts. It is so incredibly difficult to view emails with the current font. It is much too wide for quick scanning and also has a strange fuzziness to it. Stick with super accessible fonts such as Arial. Boring for some, but super accessible. I can't even use Yahoo email anymore. I use 3rd party software to view my Yahoo email. We need the option to change the font for viewing our inbox not just for writing emails. The font on your site is the same terribleness!! Why are the characters so wide?