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What is the issue?
An email is blocked in Reporting > History as 'SPF FAIL', but you still want to allow the sender through on their next attempt. This can be done by adding to the SPF FAIL exclusion list.
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What is the Solution?
Go to System Setup > Mail Authentication > SPF > SPF Bypassed IPs/Networks.
Click Add.
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In IP/Networks: enter the IP from the Client Address field (see above) in Reporting > History.
Click Save.
Further Info
When adding an exclusion as above, you can add a CIDR range by specifying the netmask value from the Netmask drop-down menu.
For more documentation, see SPF Settings.