The example comes from a mathematics lecture and was narrowed down to the problem: truncating the text behind the "less than" sign when text follows the "less than" sign without an intervening space.
 
The difference between the two lines is that "less than thirty" is once with a space and once without a space after the "less than". The forum post is sent and displayed with Thunderbird.
The Problem is described in graphics/pdf to avoid further problems.

E-Mail-Anzeige (HTML-Ansicht) (Roundcube/Thunderbird)

E-Mail-Anzeige (Plaintext-Ansicht) (Roundcube/Thunderbird)

In der Plain/Text-Ansicht are missing„<>" und „< als dreißig".