I'm using macOS Catalina (10.15.3), but this problem exists at least 5 years in the built-in mail application (Mail.app).
Mail.app in macOS seems display some incoming mails' fonts way smaller than they are supposed to be shown. In any other mail client on Windows, those mails look totally normal and the font size is well readable. Usually, it applies to personal emails formatted in RichText or HTML, that don't have particularly styled text other than some bold or colored text. It does not apply to newsletters and most generated email from online shops etc.
- One way to cope with this problem is to repeatedly press
+to increase the font size. But, as soon as you change to a different email, the size is reset to the default (which in turn is the correct behavior). When opening the same email again, you have to increase the font size all over again. It is mentioned in Mail from AOL, font to small Fix? (5 year old question with no real answer).
You can increase the overall font size for emails in settings (Fonts & Colors > Font for email). However, this will also increase the font size of subject, headers and some other text that is displayed in the preview pane for all emails, and it just looks aweful.
(Not tested) It might be possible to apply an AppleScript that modifies the email content. It is mentioned in Replace font in incoming mail [mail.app]. I can't even think that it will be successful. It might be a huge security issue.
NOT working (or no longer)
defaults write com.apple.mail MinimumHTMLFontSize 12(or 16, 18, ...)
defaults write com.apple.mail NSFontSize 12(or 16, 18, ...)
- Changing the CSS in
/Applications/Mail.app/Contents/Resources/Message.css(as mentioned in Can I increase the minimum font size of text when reading html messages in Mail.app (in Mavericks and later)?). This would also be overwritten every time Mail.app received an update.
When looking at the source of this problematic emails, I can see a lot of
<font> tags, especially parts like this, where the
size="2" might be the issue:
<font size="2">Ich danke dir!<br></font>
It used to be possible to (kind of ) fix this by editing
/Applications/Mail.app/Contents/Resources/Message.css. This file no longer exists.
Some References to this Problem on other Sites
- Apple > Support > Communities: Why is font size in email replies so tiny? (no fix)
- Apple > Support > Communities: How do I change text size in emails on OS X El Capitan?
- defaults-write.com: OS X Mail: Set minimum font size for HTML
This problem seems to exists for years and has not been addressed properly by Apple. For me, it still is the most frustrating problem after switching to the Mac from a Windows PC. Any help is appreciated!