Sendmail Setup

This is heavily based on this guide by Pritthish Nath with some slight adjustments. This guide assumes that you use DigitalOcean as your provider; however, as long as you know how to set up a PTR on your IP and set your hostnames, you'll be fine.

Prerequisites

• A VPS/Server of some kind
  • Recommend DigitalOcean
• A domain
• The Domain connected to Cloudflare nameservers and an A record pointed to your server's IP
  • Proxy is optional, but recommended
  • Cloudflare isn't required, but is recommended
• Correctly configured hosts

OpenDKIM

Installing OpenDKIM can be tricky, especially since it's kinda old, but don't worry, just follow along.

Installing

As root, simply run

apt-get install opendkim opendkim-tools

Setting up a DKIM Key

As root, run this code, replacing [DOMAIN] with your domain (include subdomain(s)!)

opendkim-genkey -t -s default -d [DOMAIN]

note

Take note of where you ran this command, as a default.txt file will be output there, which is needed in a later step

Next, lets move the key somewhere,

mkdir /etc/opendkim
cp default.private /etc/opendkim/

info

If any service asks, your Selector is default

Now, we need to give it the correct permissions

chmod -R ugo-w /etc/opendkim
chown -R root /etc/opendkim

Updating the opendkim.conf

Open the OpenDKIM config file at /etc/opendkim.conf with a preferred text editor. I'm using nano.

nano /etc/opendkim.conf

Then, replace the following values with these lines,

caution

Do not have multiple keys! If there is already a socket, either remove it, or comment out the line with a #

Domain [DOMAIN] (include subdomain(s))

KeyFile /etc/opendkim/default.private

Selector default

Socket inet:8891@[SUBDOMAIN/DOMAIN] (If your domain is something like play.hypernex.dev, then you would input play, otherwise, just hypernex. Should be a hostname)

note

Take note of your Socket value, as we will need it later

Starting OpenDKIM

Finally, start the OpenDKIM service

systemctl start opendkim

If something does not work, look for the Error status in the systemd logs

systemctl status opendkim

Sendmail

Hypernex uses Sendmail, the same as php's mail() function, to send emails. This setup is fairly easy.

Installing Sendmail

Run this command,

apt install sendmail

Configuring Sendmail

Open the Sendmail config file at /etc/mail/sendmail.mc with a preferred text editor. I'm still using nano.

nano /etc/mail/sendmail.mc

Next, add this line to the bottom, replacing the [SOCKET] with the Socket value done in the OpenDKIM configuration step from earlier.

INPUT_MAIL_FILTER(`opendkim', `S=[SOCKET]')

Finally, save and recompile Sendmail

sudo -i
cd /etc/mail
m4 sendmail.mc > sendmail.cf
systemctl restart sendmail

Adding Records

Navigate to your Cloudflare dashboard, and select the domain you will be working with, then navigate to the DNS tab.

SRV

Add a new TXT record for your domain/subdomain, and input the following text, replacing [DOMAIN] with your domain (including subdomain).

v=spf1 a:[DOMAIN] -all

Set TTL to Auto and save.

DMARC

Add a new TXT record with the following key _dmarc.[SUBDOMAIN, IF APPLICABLE], and input the following text

v=DMARC1; p=reject; sp=reject;

DKIM

Add a new TXT record with the following key default._domainkey.[SUBDOMAIN, IF APPLICABLE], and input text from your default.txt file from earlier.

Assuming your default.txt file looked like this,

default._domainkey      IN      TXT     ( "v=DKIM1; h=sha256; k=rsa; t=y; "
          "p=0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
          0000000000000000000000000000000000000000000000000000000000000000"
          "000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000" )  ; ----- DKIM key default for play.hypernex.dev

The text would be this,

v=DKIM1; h=sha256; k=rsa; t=y; p=00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

PTR Record

Assuming you used DigitalOcean, all you need to do is set your droplet's name to your domain, then reboot your droplet (assuming you changed the name). If you did not use DigitalOcean, please check with your provider on how to set a PTR record for your shared/dedicated IP address.