Mail from specific sender didn't arrive first time, but did when sent again later

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It could still be greylisting, it depends on whether or not the sender’s server is configured to resend when MIAB rejects the initial message. If the sending server’s IP isn’t already in the whitelist, you could certainly add it.

Disable greylisting/modify whitelist?

Hello. No there’s no way to disable it, sorry. To whitelist you can add the IP address or hostname of the sending mail server in /etc/postgrey/whitelist_clients. Note that it’s the sending mail server and not the domain in the From/envelope header.