Delivery-Status-Notification in sendmail 8.8 and later

Beware: this feature is undocumented in 8.8 and 8.9! It gives you a nice and simple way to ask for a DSN if your MUA doesn't currently support it. FFR means For Future Releases.

To enable the recognition of the Return-Receipt-To: header in sendmail-8.8.5 or later, do the following:

1. Add -D_FFR_DSN_RRT (for sendmail-8.8.7 and later, it's called _FFR_DSN_RRT_OPTION) to the ENVDEF line of your Makefile and recompile sendmail.
2. Add O RrtImpliesDsn=True to your sendmail.mc file in the LOCAL_CONFIG section.
3. Install both files and restart sendmail.

Since sendmail 8.10, you can use

define(`confRRT_IMPLIES_DSN', `True')

When your newly installed sendmail sees a Return-Receipt-To: header, it will treat it as if -Nsucess,delay,failure was given on the command line, or the equivalent DSN sequence was specified in the ESMTP dialog.

Note: for sendmail-8.8.7 and 8.9.x, it's called _FFR_DSN_RRT_OPTION. Thanks to Robert Sargent for pointing this out. He also noted that Makefiles/Makefile.FreeBSD doesn't have ENVDEF at all, so you need to add/change some lines:

ENVDEF=
CFLAGS+=-I${.CURDIR} ${DBMDEF} ${ENVDEF}