=== Update Version 02/24/2025

apt install opendkim opendkim-tools swaks
vi /etc/opendkim.conf
>>>>>
Add following lines towards end. Make sure you replace example.com with your domain/subdomain.

Domain                  example.com
KeyFile                 /etc/postfix/dkim.key
Selector                mail
SOCKET                  inet:8891@localhost
<<<<<

vi /etc/default/opendkim
>>>>>
Change default socket path by commenting out the current SOCKET line and making it look like below:

SOCKET="inet:8891@localhost"
<<<<<

vi /etc/postfix/main.cf
>>>>>
Add following lines towards end.

# DKIM
milter_default_action = accept
milter_protocol = 2
smtpd_milters = inet:localhost:8891
non_smtpd_milters = inet:localhost:8891
<<<<<

opendkim-genkey -t -s mail -d example.com --bits=1024
cp mail.private /etc/postfix/dkim.key

cat mail.txt
Create TXT DNS record:
>
host field: mail._domainkey.example.com
value field from the mail.txt file: 
v=DKIM1; h=sha256; k=rsa; t=y; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDPj+XGkFG/bSDQAUl3n5jHx77w0FwgtSfpeBB9k2/vkbSxGENHmwN9wm0buyIQx3w86YOWs9lLfLTVYO3+QWbfqYsfb/QsIoyf/+Pq+fwKkFVKOw9NLSpmXYdzTUwvHzmp78cCocjhPSdTSlQahyWzdW0aA2qqgOtRMvnD5Fr7RwIDAQAB
<

service opendkim restart
service postfix restart

=== You can verify your DKIM record is properly configured:
https://protodave.com/tools/dkim-key-checker/

=== set up the SPF DNS record:
https://mxtoolbox.com/SPFRecordGenerator.aspx

=== Set up the DMARC DNS value:
Sender: _dmarc.example.com
DNS Type: TXT
Value:
v=DMARC1; p=none; rua=mailto:info@example.com; pct=100; adkim=s; aspf=s

=== Set up the MX record if you haven't already done so
Type: MX
Hostname: example.com
Value: 10 example.com

=================== Other issues that might come up

dig mail._domainkey.example.com TXT

swaks -t check-auth2@verifier.port25.com -f me@example.com

Example SPF:
v=spf1 a mx a:example.com ip4:104.237.153.221 ip6:2600:3c01::f03c:93ff:fe2c:2a81 ~all