Postfix

http://www.techknowjoe.com/article/create-your-own-email-forwarding-server

https://www.digitalocean.com/community/questions/how-do-i-setup-a-ptr-record

https://www.upcloud.com/support/secure-postfix-using-lets-encrypt/

https://seasonofcode.com/posts/custom-domain-e-mails-with-postfix-and-gmail-the-missing-tutorial.html

https://serverfault.com/questions/635293/postfix-as-email-forwarder-to-gmail-spf-problems

https://www.mind-it.info/2014/02/22/forward-postfix-spf-srs/

sudo apt install postsrsd
sudo postconf -e "sender_canonical_maps = tcp:127.0.0.1:10001"
sudo postconf -e "sender_canonical_classes = envelope_sender"
sudo postconf -e "recipient_canonical_maps = tcp:127.0.0.1:10002"
sudo postconf -e "recipient_canonical_classes = envelope_recipient"
sudo nano /etc/systemd/system/multi-user.target.wants/postsrsd.service -> edit localdomain
sudo nano /etc/default/postsrsd
sudo service postfix reload
sudo tail -f /var/log/mail.log