siji/postfix repository overview

⁠Postfix

A simple SMTP Server build with Postfix + OpenDKIM + SASL authentication support.

Latest doc: https://github.com/chenzhiwei/dockerfile/tree/master/services/postfix⁠

⁠Configuration

• DOMAIN, the right part of @ in email address.
• USERNAME, the SMTP server username.
• PASSWORD, the SMTP server password.
• SELECTOR, the DKIM selector which used in DKIM DNS TXT record, default value is mail.
• TLS_KEY_FILE, the SMTP server tls key file, default to smtp.{DOMAIN}.key
• TLS_CERT_FILE, the SMTP server tls certificate file, default to smtp.{DOMAIN}.crt
• DKIM_KEY_FILE, the DKIM private key file, default to {SELECTOR}.private

⁠Generate DKIM private file

DOMAIN=youya.org
SELECTOR=mail

docker run --rm -v $(pwd):/data docker.io/siji/postfix:latest --restrict --bits=2048 --domain=${DOMAIN} --selector=${SELECTOR} --directory=/data

After this command, you will get two files mail.private and mail.txt in your current directory.

mail.txt contains something like mail._domainkey IN TXT ( "v=DKIM1; k=rsa; s=email; "xxx" ), go to your authoritative DNS server provider dashboard(such as cloudflare) to add a TXT record, the key is mail._domainkey and the value is in side the brackets.

mail.private will be used in the create container part.

⁠Generate TLS key and certificate

Suppose your domain is youya.org and you want to host the SMTP server under smtp.youya.org, then you need to apply a certificate with DNS name smtp.youya.org.

• smtp.youya.org.key
• smtp.youya.org.crt

⁠Simple Use

⁠Setup DNS records

• Setup SPF record

  key: @, value: v=spf1 a:smtp.youya.org ~all, record type TXT

• Setup DKIM record

  key: mail._domainkey, value: "v=DKIM1; k=rsa; s=email; "<REPLACE WITH YOUR OWN IN mail.txt>", record type TXT

⁠Start the Postfix container

mkdir -p /etc/postfix/secret
cp mail.private /etc/postfix/secret/mail.private
cp smtp.youya.org.key /etc/postfix/secret/smtp.youya.org.key
cp smtp.youya.org.crt /etc/postfix/secret/smtp.youya.org.crt

docker run -d --name postfix -p 587:587 \
    -v /etc/postfix/secret:/etc/postfix/secret \
    -e DOMAIN=youya.org \
    -e USERNAME=your-name \
    -e PASSWORD=your-pass \
    docker.io/siji/postfix:latest

⁠Send the email

Create a file named email.txt with following content:

From: [email protected]
To: [email protected]
Subject: an example email

Dear Joe,
Welcome to this example email. What a lovely day.

Send it:

curl -v --ssl --url smtp://smtp.youya.org --mail-from [email protected] --mail-rcpt [email protected] --upload-file email.txt

A few seconds later, you will receive an email with title Authentication Report and it shows your SMTP server result.