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From Wikipedia:
Sender Policy Framework (SPF) is a simple email-validation system designed to detect email spoofing by providing a mechanism to allow receiving mail exchangers to check that incoming mail from a domain comes from a host authorized by that domain's administrators. The list of authorized sending hosts for a domain is published in the Domain Name System (DNS) records for that domain in the form of a specially formatted TXT record. Email spam and phishing often use forged "from" addresses, so publishing and checking SPF records can be considered anti-spam techniques.
To add a SPF record in your DNS, insert the following line in your DNS zone:
; Check that MX is declared
domain.com. IN MX 1 mail.domain.com.
; Add SPF record
domain.com. IN TXT "v=spf1 mx ~all"
This enables the Softfail mode for SPF. It is a good setting for getting started and testing everything. After verification, you might want to change your SPF record to: v=spf1 mx -all, to enforce the policy!
Increment DNS serial and reload configuration.
Backup MX, Secondary MX
For whitelisting a IP-Address from the SPF test, you can create a config file(See policyd-spf.conf) and mount that file into /etc/postfix-policyd-spf-python/policyd-spf.conf
Example:
Create and edit a policyd-spf.conf file here /<your Docker-Mailserver dir>/config/postfix-policyd-spf.conf:
debugLevel = 1
#0(only errors)-4(complete data received)
skip_addresses = 127.0.0.0/8,::ffff:127.0.0.0/104,::1
# Preferably use IP-Addresses for whitelist lookups:
Whitelist = 192.168.0.0/31,192.168.1.0/30
# Domain_Whitelist = mx1.mybackupmx.com,mx2.mybackupmx.com
Then add this line to docker-compose.yml below the volumes: section
- ./config/postfix-policyd-spf.conf:/etc/postfix-policyd-spf-python/policyd-spf.conf