I recently received a message marked as spam that causes the above assertion when parsing.  However, it only occurs when the message is parsed with GMimeStreamCat (the header buffer from one stream, the body buffer from another).  If they're loaded into a single buffer and parsed, there's no problem.

The broader issue is that I run with fatal-warnings, and so this causes Geary to crash for me every time.

I'm concerned about attaching the message directly here, as it's spam and has a ZIP attachment that holds a .EXE.  If someone wants to see the message, I can email it directly.

I have a Vala program which reproduces this problem.  It looks like this:

void main(string[] args) {
    GMime.init(0);
    
    try {
        string header;
        FileUtils.get_contents("header.txt", out header);
        
        string body;
        FileUtils.get_contents("body.txt", out body);
        
        GMime.StreamCat stream_cat = new GMime.StreamCat();
        stream_cat.add_source(new GMime.StreamMem.with_buffer(header.data));
        stream_cat.add_source(new GMime.StreamMem.with_buffer(body.data));
        
        GMime.Parser parser = new GMime.Parser.with_stream(
            stream_cat);
        parser.construct_message();
    } catch (Error err) {
        stderr.printf("%s\n", err.message);
    }
}

... where header.txt has the headers and body.txt the body.