List comprehensions provide us with a simple way to create a list based on some iterable. During the creation, elements from the iterable can be conditionally included in the new list and transformed as needed.
Have you error 'ssl_stapling_init_cert: Can't retrieve issuer certificate!' in your apache log file? Or error 'OCSP_basic_verify() failed (SSL: error:27069065:OCSP routines:OCSP_basic_verify:certificate verify error:Verify error:unable to get local issuer certificate) while requesting certificate status' in your nginx error file? Probably you have a misconfigured web server for OCSP STAPLING support
Sender Policy Framework (SPF) is an email authentication method designed to detect forged sender addresses in emails (email spoofing), a technique often used in phishing and email spam. SPF allows the receiver to check that an email claiming to come from a specific domain comes from an IP address authorized by that domain's administrators. The list of authorized sending hosts and IP addresses for a domain is published in the DNS records for that domain. SPF together with DKIM are based stones to DMARC