I'll leave the subject line substitution issue to Robin's superior expertise...
For the message body, sorry, I should have typed 100kB, as in 100,000 Bytes. We haven't identified an ~exact~ number of characters it will tolerate, just noticed that strange things start happening when the raw data going into that field gets near the 100kB mark. If you can save your message text in a Notepad .txt file, and if the resulting file size weighs less than 100kB (and the further below that mark, the better), then you should be fine pasting that text into the message body field.