You might try this..
In the script step to send email from fileMaker, you can set the message
Use a calculation to remove the text formatting:
TextFormatRemove ( FieldNameOrText )
This should send the text as raw text, not as the format the filemaker field.
If you insert the required html tags into it (start it with "html" (enclosed within chevron brackets), end it with "/html" and mark out the bold sections with "b" and "/b", then save it to disk like that and send it as an attachment, it will carry the formatting with it.