Change "Mail Delivery System "

Too lazy to look at the last links to the DA help sections?

@Zeiter: When doing the "bounce_retu_message = false", must this be done in /etc/exim.conf or can this also be placed within /etc/exim.variables.custom.conf?
And if yes, does one need to put:
bounce_retu_message = false
or
bounce_retu_message == false
in the custom file?