From your description, the issue may just be a slow, intermittent, internet connection. I have a particular email account that does the same thing, saving sometimes and sometimes not. The error message's words, as I recall, may be exactly the issue. All Thunderbird can do is take your message and 'toss' it to the server, but it's up to the server to 'catch' it. I do appreciate the frustration, but knowing how to troubleshoot an internet connection is far beyond my few talents.