Q0738 - Exim/exim GitHub Wiki
How can I configure Exim to delay the SMTP connection if more than 10 invalid recipients are received in one message?
Put something like this in your RCPT ACL:
deny message = Max $rcpt_fail_count failed recipients allowed
condition = ${if >{$rcpt_fail_count}{10} {1}}
! verify = recipient
delay = ${eval: $rcpt_fail_count * 10}s
log_message = $rcpt_fail_count failed recipient attempts
This example increases the delay for each failed recipient.