K-9 Mail: Search in Message
After an Update of K-9 Mail I tried again to find it and succeeded (don't know whether this worked in a version prior to the new one).

This is how you search in K-9 Mail inside a message: In the respective message


select any text;
in the upcoming menu (copy, past, select all), select the document with a magnifying glass on it;
now the text you selected is used for an in-message search;
edit the search box and enter your search string.