The problem with Thunderbird, and other "exotic" email clients is that very few people seem to use them in the corporate world. They are the domain of "geeks", the sort of people who prefer Unix to Windows. (Some of us have fond memories of Eudora!) So there is no pressure on X1 from its main customer base, big business, to support Thunderbird. Some corporates might actually lock down their PCs and ban employees from using Thunderbird on company machines.
Another issue is the components that X1 buys in as helpers. The viewers, for example, come from Oracle. That too has little interest in the mass market, so we can't have viewers for OneNote, Evernote and other tools that make life easier for the rest of us.
Another issue is the components that X1 buys in as helpers. The viewers, for example, come from Oracle. That too has little interest in the mass market, so we can't have viewers for OneNote, Evernote and other tools that make life easier for the rest of us.