Paul Smith wrote:
It wouldn't even need to be much, just respond
421 4.2.x [retry:300s] connection refused for now. Try again later
it wouldn't interfere with MTAs which don't understand it, and would
give a hint to those which do.
FWIW, Here are some forms of apparent GL responses with hints I have
collected:
421 This server implements greylisting, please try again in # seconds
450 4.7.1 <RCPT>: Recipient address rejected: Greylisted for # minutes
450 4.7.1 <RCPT>: Recipient address rejected: Greylisted for # seconds
451 4.7.1 Greylisting in action, please come back in HH:MM:SS
451 Greylisted for # seconds
451 Greylisted, please try again in # seconds
451 Greylisting enabled, try again in # minutes
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HLS