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Grey listing is simply an older form of anti-spam.
Grey listing typically refuses the first connection from an unkown sending ip and usually caches the ip for a specific time to see if there is another connection which then verifies the sending ip.
The ironmail by default waits 15 minutes to send the first retry, which should then be accepted by the recieving server.
Will it be better to reduce the waiting time from 900 sec to 400 sec ?