This Knowledge Base Article provides possible workarounds to the issue met when greylisting is enabled on the server side and when the email proxy on the XG is configured to Legacy Mode
You will see the following error message in your logs when faced with this problem: "451 Temporary local problem, please try again!".
This is due to Legacy Mode not supporting e-mail retry.
Two workaround options are provided along with possible side effects.
Cheers,
Karlos
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