imagehtmlemail sending as text/plain

we have some code that uses imagehtmlemail, from apache commons. From most machines it works perfectly, sends out a html email with an embedded image - but from some machines - with exactly the same code, exactly the same smtp server, and inside the same networks - it sends the multipart email as normal - but each of the parts is marked as text/plain - as opposed to having a text/html and text/plain section when it works. And of course the users get a page full of html markup as the result.

I can't see how this is possible. In code we are using setHtmlMsg of course - and from other machines the same code works - so it must be something environmental or configuration on the machine itself - but nothing I can determine. Any ideas of either what could be the problem, or what I should check?