Assumptions

You are using 3rd party email service provider (like Gmail, Outlook, ProtonMail, iCloud, etc), you have a server capable of running Postfix and you want to have the capability to send transactional emails while preserving your 3rd party service for incoming emails.

No Guarantees

It takes a lot of work to set up a deliverable email server. For me, the setup I described here works perfectly - my emails weren't blacklisted or automatically classified as spam. I was able to deliver them to all major email service providers successfully.

Remember that your results may vary; many factors contribute to deliverability, and I'm not responsible if something goes wrong. You have been warned.

The Goal

Our final goal is to be able to send transactional emails from our server and still be able to both receive and send them through 3rd party service provider. Let me explain.

Postfix is an open-source email transfer agent. It can act both as an SMTP server and client, but we will only use its server capabilities.
When it comes to the client part, you can use any email service provider.

Setting up an email server is unreasonably burdensome because you must handle the incoming spam and storage. It's much easier to rely on an external email provider. On the other hand, free email providers are unsuitable for sending transactional emails.

What if we could combine the best parts of both worlds and be able to send transactional emails through our server and receive them through a 3rd party client?

Postfix

Installation

There are multiple ways to install Postfix, depending on your operating system so that I won't be covering that here. I'll assume you have it running.

It's pretty simple; for example, on a Debian-based system, it's usually just a matter of:

$ sudo apt install Postfix

Configuration

To deliver your emails to most inboxes, you need to enable TLS email encryption in your Postfix server. To do so, you need to add the lines:

smtpd_tls_security_level=encrypt
smtpd_tls_loglevel = 1

smtp_tls_security_level=encrypt
smtp_tls_loglevel = 1

*_loglevel setting is optional to add; it just helps with debugging potential issues.

Also, you need to make sure that myhostname variable is set accurately to either the current server IP or its reverse DNS address. That is crucial for setting up SPF.

DNS Setup

DKIM Record

To make your emails deliverable, you need to create a DKIM record for your server (see Mail to learn how to generate the encryption keys):

Type:  TXT
Name:  YOURDKIMSELECTOR._domainkey
Value: v=DKIM1; k=rsa; p=YOURPUBLICKEY

YOURDKIMSELECTOR will be used when sending emails to tell the recipient to validate them against the server we are currently setting up.

YOURPUBLICKEY is the public key with newlines and the envelope removed (without the -----BEGIN PUBLIC KEY----- and -----END PUBLIC KEY----- parts - or similar). Your Resonance application will have to use the same key pair you are using here to encrypt emails.

DMARC Record

You most likely already have DMARC record configured, as most 3rd party email providers require it. It should point to an email that handles potential deliverability issues. For example:

Type:  TXT
Name:  _dmarc
Value: v=DMARC1; p=none; rua=mailto:postmaster@YOURDOMAIN

Replace YOURDOMAIN with your actual domain.

SPF Record

SPF records tell what servers can send emails from your domain.

You probably already have a configured SPF record from your email provider. For example, if you are using Protonmail you might have a record like this:

Type:  TXT
Name:  @
Value: v=spf1 include:_spf.protonmail.ch mx ~all

You need to change it to incorporate your server's IP address (you can also delegate that to another record by using include:). For example:

Type:  TXT
Name:  @
Value: v=spf1 include:_spf.protonmail.ch ip4:YOURSERVERIP mx ~all

Replace YOURSERVERIP with your actual server IP address.

By setting up the SPF record like above, you will be able to use both your email provider and your server to send emails.

Resonance Setup

Once you have both Postfix and DNS records set up, you can configure Resonance to handle outgoing emails through Postfix.

Configuration

Add Postfix section to your mailer (you can name it any way you want; it doesn't have to be postfix):

; ...

[mailer]
postfix[transport_dsn] = native://default
postfix[dkim_domain_name] = YOURDOMAIN
postfix[dkim_selector] = YOURDKIMSELECTOR
postfix[dkim_signing_key_passphrase] = YOURPASSPHRASE
postfix[dkim_signing_key_private] = dkim/private.key
postfix[dkim_signing_key_public] = dkim/public.key

; ...

Replace YOURDOMAIN, YOURDKIMSELECTOR, YOURPASSPHRASE with appropriate values.

Sending Emails

You can refer to Mail documentation to learn how to send emails with Resonance.

You should be able to immediately send a deliverable email by using the command line:

$ php bin/resonance.php mail:send \
    --from "me@YOURDOMAIN" \
    --to "you@YOURDOMAIN" \
    --subject "Hello!" \
    --transport "postfix" \
    "How is it going?"

Or programmatically:

/**
 * @var Distantmagic\Resonance\MailerRepository $mailerRepository
 * @var Symfony\Component\Mime\Email $email
 */
$mailerRepository->mailer->get('postfix')->enqueue($email);

Remember to warm up your IP address and domain before lots of transactional emails.

Summary

By now, you should have a transactional email server with its own dedicated IP address and still retain the possibility of receiving incoming emails in the same inbox you have used and loved. :) Congratulations!