We're using the CMU Cyrus Sieve implementation. A few of our more advanced
users have asked how to identify bounce messages in a Sieve script. The
obvious doesn't work:
if envelope :is "from" ""
The best phrasing we have seen is
if not envelope :contains "from" "@"
which is a rather ugly circumlocution. RFC 3028 doesn't say anything about
null return paths, which is clearly an omission. As a result the Cyrus
behaviour is hard to classify as a bug.
Tony.
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