Sieve reject with ORIG-TO vs TO

[E.B.]

Hi,

The bounce message generated by the reject extension
has what looks like a hard coded message prefix that
comes before the configurable reason text:

"Your message to <username> was automatically rejected:"

In some cases, the <username> is NOT the original-to
address, which can cause confusion to the sender or
expose private aliasing data that some people might
want to hide.

Is there a way to make the reject extension use the
original-to address in that hard-coded message prefix?

Is there a way to completely customize the FULL 
message?

Is there a way to customize the headers for such
messages?