To format text, such as changing the font size or colour or inserting smileys, you need to have the text editor set to “HTML”. You change this while composing an email by setting the “Editor type” to “HTML” (on the left, at the top of the text box). You will then have a list of symbols above the text box for formatting text.
Via “Settings” > “Preferences” > “Composing messages” you can set “Compose HTML messages” so that new messages will be in HTML by default, or alternatively, only when replying to a message in HTML. By changing the default settings here, you won’t have to select “HTML” in the text editor every time.
Please be aware of the following: Changing the font display options is not really necessary for emails and can lead to problems for the recipient. This is not specific to Posteo, but general advice. A smartphone, for example, will work out an optimal display for its screen size by itself – but with manual formatting from a user it makes this process more difficult or impossible, and can even result in text being unreadable. We recommend use of HTML for short passages only, for example, to make text bold, etc.