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If you notice that outbound emails from your server are taking hours to send, it may be caused by the blockcracking configuration. This is a very easy fix.
Step 1: Open /etc/exim.blockcracking/variables.conf.custom
Step 2: Add this to the file:
BC_VERIFY_CALLOUT ==callout=10s,maxwait=40s,connect=5s,defer_ok,use_sender
Step 3: Restart exim with:
systemctl restart exim