On 06/12/2018 00:02, Jasen Betts via Exim-dev wrote:
> NDRs go to the envelope sender, so they will bounce back to a single
> address (per message), but there can be several "delayed" messages and
> a single bounce (retry timeout exceeded) for each input, so that
> provides small-scale amplification, until the timeout is reached,
> after that no amplification.
> where available SPF is one mitigation for this. It prevents the attacker 
> from forging the sender address.

Alternatively, set delay_warning_condition to only send warnings if the
email was sent by one of your own users.

Simon Arlott

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