This is, unfortunately, a common confusion, as there are actually two different settings for changing your font style in eM Client. One is your preferred style for reading messages and the other for writing/composing a message.

1. Menu > Settings > Mail > Read

The Preferred Style setting in this section will be applied to the messages you read, if they use Plain text. Plain text format does not have any set "visual" formatting, so you can apply your own.
This way you can make the font easier to read for you.

The setting will not be applied to HTML messages, which do have set formatting.

2. Menu > Settings > Mail > Compose

The Preferred Style in this section will be used for writing all your messages.