I recall issues where ther were large emoji's in the Subject, so perhaps this is the same issue, but in content. It only seemed to effect Linux OS. https://support.mozilla.org/en-US/questions/1178430 PXFX2 says: ''OK, solve it. Well maybe a bug in TB, using Googles notoColorEmoji.ttf (https://github.com/googlei18n/noto-emoji) installed in .font folder (linux here) and having no other emoji style font files installed, notoEmoji-Regular.ttf or Symbola-emoji.tff causes this issue. installing one of the standard non-colour emoji fonts and the problem goes away, and so does your coloured emojis. TB seems to use the standard version in perference to the color version font file'' pjborad says: ''This work for me thanks. I had to place the file notoColorEmoji.ttf into the directory /usr/share/fonts/truetype/noto though. I'm running Ubuntu Linux 18.04 (dev) with Thunderbird 52.6.0 .'' Hope the above info helps you. https://bugzilla.mozilla.org/show_bug.cgi?id=1458797