RFC 5321 §4.5.5 states:
All other types of messages (i.e., any message which is not required by a Standards-Track RFC to have a null reverse-path) SHOULD be sent with a valid, non-null reverse-path.
For the avoidance of doubt, RFC 2119 §3 defines "SHOULD" as "there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course."
The "particular circumstances" under consideration are where Alice does not wish to receive delivery status notifications (related to emails that she originated).
She has a few options:
ignore/discard any such notifications when she receives it—clearly this behaviour would be entirely correct, but it's a nuisance (and a waste of her resources);
refuse to accept such notifications at some lower level—arguably the reverse-path would no longer be "valid" (sadly not defined in the RFC) and, if so, such behaviour is contrary to the RFC's recommendations; or
originate the email with a null reverse-path so that notifications never get generated in the first place—clearly contrary to the RFC's recommendations.
I'm pretty sure that these circumstances constitute a "valid reason" to ignore the RFC's quoted recommendation above—but I want to be sure that I've understood the "full implications" of options 2 and 3.
In particular, I note that:
Under option 2, some agents may detect that the reverse-path is not valid
Under option 3, some agents may treat mails with null reverse-paths as special cases
Either way, the agents might consequently refuse to accept/relay/deliver the email, or may use such information to increase the likelihood of its being flagged as spam. How common are such measures in practice?
Are there any other concerns/implications that I should consider?