OK, here is the email exchange with a tech support dude:
A friend gets your emails and if the text in a paragraph is too long, it gets truncated. He is not very computer-literate and asked me to help figure out how to get it fixed. Here is a recent example:<example not included here but it showed the problem>
It's caused by his email client. We send the text as a text file with only carriage returns between paragraphs. This way people can format it how they wish. The problem is some mail clients truncate the long paragraphs.
So, I am left with the distinct impression that too many characters in a row without a carriage return will cause that version of AppleMail to barf as it gets to the maximum number of characters. Not quite a "buffer overflow", but a similar concept.
I have never seen any mail client on any platform truncate paragraphs. The worse that happens is that some clients will insert LF/CR to enforce a line character limit--usually 72 characters per line. I no longer use Mail 1.2.5, so I am not able to test my only theory about what you are seeing. That is, for some reason your paragraphs may not be properly soft-wrapping to fit the message window. I have seen this while using a text editor. However the entire message should still be there. Drag your horizontal scroll bar to the right.
Oh, I agree. It bloody well should be there, but it ain't. The program stops accepting characters at 999 and won't start again unless it gets a carriage return, which it does at the end of the paragraph.