The To/CC/Subject etc. are part of the message header where there is no formatting, so any formatting you did choose for those fields when composing the message would not follow with the sent message. It just depends on which font the UI uses as to how they are displayed.
You might want to look into editing themes in eM Client, if you want to change the way the UI is displayed on your computer, but I really don’t know if you can change the fonts for those things.
Tks Gary. That’s kind of weird I’m using the default theme only n didn’t change a thing, but the fonts in the to/subject field looks totally different *_*