OK, further tests have shown there is a problem.
The problem appears to be a bug in PHP.
We've put in a workaround for that problem, but then we also decided to handle header line folding (76 chars is recommended but not required by the internet standards).
The folding then caused problems with the quoted printable encoding.
We're now working on resolving these.
Version 8.22 of FormMail will be released as soon as everything is beta tested and this should resolve all these utf-8 problems.