- https://en.wikipedia.org/wiki/Greylisting_(email)
- https://de.wikipedia.org/wiki/Greylisting
- https://wiki.hostsharing.net/index.php?title=Greylisting
- https://doc.hostsharing.net/referenz/domain/domain-optionen.html : Package > … > Domain > (domain name) > Domain options > [_] greylisting
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