Configurable validation of SMTP recipients about notqmail

[schmonz]

Andrew Richards's qmail-verify is a practical improvement on realrcptto: the recipient-validation logic moves into a new service that runs as root, so it produces more accurate results than qmaild can. I should probably add a qmail-rcptcheck-verify program to rejectutils.

Wild speculation about config: when we have extensions, it might be sensible for us to ship the qmail-verify daemon and recommend enabling qmail-rcptcheck-verify. It's qmail's own delivery logic, after all.

Wild speculation about design: maybe we can avoid duplicating the logic. Instead, extract the originals to objects that can be linked into their original call sites and also into qmail-verify.

Caveat: though realrcptto has been made to work with qmail-qmtpd, qmail-verify has not, as yet.

[DerDakon]

I have patches at my qmail patch site that do that verification if you use vpopmail and have the users in the filesystem (in contrast to a database). Similar code is in Qsmtpd which I have in production for more than a decade and it just works. It detects the .qmail-default catchalls and rejects, and it also works with sub-addresses of users. I'm not 100% sure for the naked qmail patch, but that can probably be adapted to the latest code I have in Qsmtp.

[schmonz]

I think we can ship a decent solution relatively cheaply by repurposing existing code, so I’m suggesting this as one of the small number of new features for 1.09 and will submit a PR soonish.