For Windows, there’s no EMail setup options in Internet top-level configuration category. Sending is done using LibreOffice’s
senddoc.exe program, that in turn simply uses Windows MAPI dll (mapi32.dll; see senddoc.cxx:259 and simplemapi.cxx:30). The Windows dll’s
MAPISendMail function does all actual job. So, the failure to find the mailer happens outside LibreOffice’s code. I only could guess that LO/TB bitness might have something to do with this problem.
It’s unhelpful to double-post. If you really need to send your question again, then at least remove your previous one, to keep things clean. Or, better, just update your initial question with additional data (e.g., what you tried in between).
[EDIT 2017-07-31]: There is a Thunderbird problem that prevents “Send to > Mail recipient” function to work correctly (see e.g. TB release notes for 52.2.1,
Known Issues). The workaround mentioned there is to install Microsoft Visual Studio 2015 redistributable runtime library or the Universal C Runtime for Windows Server. Maybe it’s something related to this issue as well.
[EDIT 2018-03-09]: LibreOffice had used 8-bit MAPI to communicate with default mail provider; it also used original filename for temporary attachment files. With some of recent changes I made to the code (starting with this commit, followed by this, this, and this), it now uses 16-bit Simple MAPI, which eliminates some of problems that could result in such a message. Also, a problem code is now shown in the error message, to help determine the real problem. It will be available in version 6.1 (or using current dailies).
Specifically, this kind of problem could, among other reasons, be triggered when the document being sent contained some characters in name which couldn’t be represented in current 8-bit Windows codepage. To check the problem, just create a simple file named “aбγ.odt” (i.e., containing Latin, Cyrillic, and Greek in the name - so cannot be represented in any Windows ACP), and try to sent it.