Why does Mail gets truncated for no apparent reason?

I composed an email in Word (on my MacBook Pro) then copied pasted it into an email (ro avoid using attachments)

Something caused the message to be truncated about one third of the way through

This happeed again when resent to the saame addressee

I dropoped off the first part of the message (that had been previousy transmitted ahd received) and it worked

with thensecond part!

This is the second time this has happened in similar circumstances

Could a spam filtermisdoagnosed a problem? There was no appparent reolationship with the point of trubcation

The text had a couple of links. Could this have caused the hiccup? There was no relationship to the placement of the cut

The text was quite long but had no images or special characters - other than the two HTML links

Puzzled and concerned