OK, here is the email exchange with a tech support dude:
From me:
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>
From him:
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.
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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.
Fred