How to configure Symfony Mailer

Symfony applications need a Mailer transport before password resets, notifications, registration messages, or background email jobs can leave the app. The transport DSN tells Symfony Mailer whether to use an SMTP server, a provider bridge, sendmail, a local catcher, or a disabled transport for a test environment.

The default Symfony Flex recipe stores the active transport in MAILER_DSN and loads it through the Mailer package configuration. Keep local overrides outside committed defaults, and keep production credentials in deployment configuration or Symfony secrets rather than committed dotenv files.

A local Mailpit catcher lets the test message be inspected without reaching a real inbox. Replace the DSN with your provider's supported format for production, and URL-encode usernames, passwords, or API keys that contain characters such as +, /, @, or :.

Steps to configure Symfony Mailer transport:

Confirm the Mailer package configuration reads the DSN from MAILER_DSN.
```
$ cat config/packages/mailer.yaml
framework:
    mailer:
        dsn: '%env(MAILER_DSN)%'
```
Symfony loads package configuration from config/packages. The Mailer recipe normally creates this file when symfony/mailer is installed.
Start a local Mailpit SMTP catcher for the configuration check.
```
$ docker run --detach --name mailpit --publish 2525:1025 --publish 8025:8025 axllent/mailpit
26f15f3718905bad5bda0ccd69e969e040b82c3afff6fd4fb9f25368cbabb7aa
```
The command maps host port 2525 to Mailpit's SMTP port and exposes the web/API inbox on http://127.0.0.1:8025. Use an existing catcher instead if your project already has one.
Add the local SMTP transport DSN to .env.local.
```
# .env.local
MAILER_DSN=smtp://127.0.0.1:2525
```
Do not commit .env.local when it contains real SMTP credentials or provider API keys. Store production secrets in the deployment environment or Symfony secrets instead.
Related: How to set Symfony secrets

Confirm Symfony reads the active Mailer DSN.

$ php bin/console debug:container --env-vars
Symfony Container Environment Variables
=======================================

##### snipped #####
  MAILER_DSN                n/a                "smtp://127.0.0.1:2525"
##### snipped #####

The Real value column is the value available to CLI commands. Web requests can differ when the web server sets environment variables separately.
Related: How to set Symfony environment variables

Send a test message through the configured transport.
```
$ php bin/console mailer:test ops@example.com --from=app@example.com --subject='Symfony Mailer check' --body='Mailer transport is configured.'
```
mailer:test sends directly through the Mailer transport and bypasses Messenger when async email delivery is configured.

Verify the message reached the local catcher.

$ curl -sS http://127.0.0.1:8025/api/v1/messages
{
  "total": 1,
  "messages": [
    {
      "From": {"Address": "app@example.com"},
      "To": [{"Address": "ops@example.com"}],
      "Subject": "Symfony Mailer check",
      "Snippet": "Mailer transport is configured."
    }
  ]
}

A message with the expected sender, recipient, subject, and snippet shows Symfony loaded the DSN and handed the email to the selected SMTP transport.

Stop the local catcher when the configuration check is finished.
```
$ docker rm --force mailpit
mailpit
```
Leave Mailpit running if it is the development catcher for the project.