If the sender sent email as a plain text email then that is all you are going to see. The colour of the text will be whatever you have set up to use. The default is black text and white background, but this may be different depending upon your settings and whether you use a userContent.css file.

You can check to see what is in the email source. Select the email so it displays in Message Pane and click on 'More' button located in the header area. select 'View source' from drop down list. It will open in a new window. All headers are displayed first followed by : Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit followed by content of email. If html has been included by sender it will be displayed below as: Content-Type: text/html; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit followed by html code and content.

If you have selected: View > Message Body As > Original HTML Then the Content-Type: text/html section is displayed, but if it is not present in email then Content-Type: text/plain information is displayed.

Thanks for your answer!

If I understood you correctly, text written without formatting will just be displayed as plain text. But how is it then that in other e-mail programs the text is formatted e. g. with the font Calibri? Is there a similar option in Thunderbird?

Thanks in advance!

Thunderbird has default display fonts for plain text. They are set in options.

Perfect! I have now found the related options and could change the appearance.

Thanks for your help!