To keep spam lower, add a per domain feature (may be with flags to be enabled or not) to:
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Reject emails with no DMARC configuration (no spf, no dkim signature). May be too strict but google is going to implement it.
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Defer emails with SPF temp error
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Reject based on several good/reliable dns public RBLs with no false positives like like zen.spamhaus.com, Barracuda, Hostkarma black, Spameatingmonkey (that also has a Fresh domains list to reject newly registered domains). May be pre-configured lists and enabled by flags.
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Reject if HELO or mail-from or From domain is listed in URIBL black (dns list) or may be other good dns lists like Spamhaus DBL, Spameatingmonkey (sem-uri or sem-fresh10).
To keep spam lower, add a per domain feature (may be with flags to be enabled or not) to:
Reject emails with no DMARC configuration (no spf, no dkim signature). May be too strict but google is going to implement it.
Defer emails with SPF temp error
Reject based on several good/reliable dns public RBLs with no false positives like like zen.spamhaus.com, Barracuda, Hostkarma black, Spameatingmonkey (that also has a Fresh domains list to reject newly registered domains). May be pre-configured lists and enabled by flags.
Reject if HELO or mail-from or From domain is listed in URIBL black (dns list) or may be other good dns lists like Spamhaus DBL, Spameatingmonkey (sem-uri or sem-fresh10).