Walter Dnes asked,
| Thank you very much. One question just occurred to me. Will
| a message with 1 (or more) attachment(s) be handled properly ?
If the entire message is MIME-encoded in order to carry an attachment,
probably not. Procmail will see the entire body as one piece and not
care how (nor even whether) MIME-compliant MUAs will divide it for
presentation. If you want to add something at the beginning of the first
part of a multipart message, you'll need to insert it after the first
part's own inner headers.
That is highly non-trivial. We have to look for a boundary marker, all
inner headers (including continuation lines) of the first part, and then
a blank line that starts the first part's body. Then we have to make
sure it isn't done to any subsequent part. I don't know nearly enough
about MIME to write bullet-proof sed code for that.