How mail works

Every site can send mail from the moment it exists. wp_mail() and every plugin built on it work without an SMTP plugin, through one mail relay on each server.

Mail → Overview with the checks of each server's relay and the volume each site sent

How a message leaves a site

PHP’s mail() hands the message to msmtp inside the site’s container.
msmtp logs in to the relay of the site’s server with the site’s own login, <slug>@wpl7. The connection stays on the site’s private network.
The relay checks that the site owns the sender’s domain. A signer then adds a DKIM signature when the domain has a key.
The relay delivers the message, or queues it and tries again later.

The relay publishes no port, so nothing on the internet can send through it. It also sees every message the sites on its server send, which is what makes the traffic view complete.

DKIM keys belong to the panel, and every server gets a copy of every key. A site that moves to another server keeps signing with the same published key.

Who may send as which domain

A site’s login owns the site’s domains and its dev address. The relay holds the sender address to that list, and the signer holds the From: header to it. This sender authorization keeps a hacked site from mailing as another customer.

A site sends as	The relay	The signature
One of its own domains, or its dev address	Accepts it	Added when the domain or a domain above it has a key
A domain of another site	Refuses it with a `553` error	None
A domain with a DKIM key but no site	Refuses it	None
A domain no site on your servers uses	Accepts it	None

A message with the site’s own sender address but another site’s domain in From: is delivered unsigned. The panel’s own mail, such as alerts and test messages from the relay, is handed over inside the relay and needs no login.

Direct delivery or a smarthost

By default each relay delivers to the recipients’ mail servers itself. With a smarthost, it hands every message to one SMTP provider account instead.

	Direct, the default	Smarthost
In `deploy/.env`	`SMTP_RELAYHOST` empty	`SMTP_RELAYHOST`, `SMTP_USERNAME`, `SMTP_PASSWORD`
Outbound port 25	Must be open at your server’s provider	Not needed
SPF names	Every server’s address	Your provider’s servers
Reverse DNS	Must match the mail hostname	Less critical

Use a smarthost

In /opt/wpl7/deploy/.env on the server, set the provider’s host, port and account:

SMTP_RELAYHOST=[smtp.example.com]:587
SMTP_USERNAME[email protected]
SMTP_PASSWORD=the-provider-password

Apply the change:
Terminal window
```
sudo /opt/wpl7/provision/setup.sh
```
Do the same on every server. Each relay reads its own deploy/.env.

The server’s card on Mail → Overview then shows the provider next to the server’s name.

What the Mail page shows

Tab	What it is for
Overview	Each server’s checks, a test message, and what each site sent in the last day
Setup guide	Every record and server setting delivery needs, checked against public DNS
Traffic	Every message, one row per recipient
Queue	What the relays are still trying to deliver
DKIM & DMARC	One block per sending domain, with its key and its three records

Check	What it says
`relay`	The relay runs. When it does not, `mail()` fails for every site on that server.
`dkim`	The signer runs, and signs for the domains the server’s sites send from. Red when the signer is down while keys exist.
`hostname`	The name the relay announces to other mail servers.
`mode`	Direct delivery, or the smarthost it relays through.
`milter`	The relay passes mail through the signer.
`port-25`	Direct delivery only: whether outbound port 25 is open.
`queue`	How many messages wait, and how many are deferred. Red from 50 deferred.

Edge The dkim check turns amber while sites send from a domain that no key signs, its own or one above it. It names those domains, and their mail still goes out, unsigned.

Send a test message sends one From the relay, skipping WordPress, or From a site (wp_mail), the way a contact form sends. When the relay’s message arrives and the site’s does not, the problem is in that site, such as a plugin that replaces wp_mail().

Limits

WPL7 sends mail and does not receive it. Mailboxes for your customers’ domains live elsewhere.
A relay that stops takes mail() away from every site on its server. Mail never fails over to another server.
Mail from a domain without a DKIM key goes out unsigned. Mail from a domain no site uses is accepted and never signed, and the recipient’s own checks judge it.
The smarthost is set per server. The panel does not copy it to the servers it adds.