Monolog: Send error logs by mail causes an extra mail for every error sent

Question

For every error log sent by mail we get another extra email with 2 lines of smtp debug log messages.

• Newly created symfony demo (symfony new --demo)
• Configured monolog to send error messages by mail as in the docs
• symfony/monolog-bundle 3.7.0
• Symfony Versions 5.2.1 / 5.2.9 / 5.3.0-RC1,
• PHP 7.4.13

# config/packages/prod/monolog.yaml
monolog:
    handlers:
        main:
            type:         fingers_crossed
            action_level: critical
            handler:      deduplicated
        deduplicated:
            type:    deduplication
            handler: symfony_mailer
        symfony_mailer:
            type:       symfony_mailer
            from_email: 'error@example.com'
            to_email:   'error@example.com'
            subject:    'An Error Occurred! %%message%%'
            level:      debug

Monolog sends error messages as expected, but every mail is followed by a second one with this content:

[2021-05-26T10:49:47.683298+02:00] app.DEBUG: Email transport "Symfony\Component\Mailer\Transport\Smtp\SmtpTransport" stopping [] []
[2021-05-26T10:49:47.722980+02:00] app.DEBUG: Email transport "Symfony\Component\Mailer\Transport\Smtp\SmtpTransport" stopped [] []

• no extra mail in dev or test environments
• no extra mail when setting the framework config test: true in config/framework.yaml

Any ideas how to get rid of this extra email in production mode?

PS: There's also an open issue in the Symfony MonologBundle.

Answer 1

As mentioned in the linked GitHub issue by @raziel057, a workaround would be to exclude the "mailer" channel.
https://github.com/symfony/monolog-bundle/issues/405#issuecomment-1069040234

monolog:
    handlers:
        main:
            type:         fingers_crossed
            action_level: critical
            handler:      grouped
            channels:     ["!mailer"]
        grouped:
            type:    group
            members: [streamed, deduplicated]
        streamed:
            type:  stream
            path:  "%kernel.logs_dir%/%kernel.environment%.log"
            level: debug
        deduplicated:
            type:    deduplication
            handler: symfony_mailer
        symfony_mailer:
            type:       symfony_mailer
            from_email: "%mailer_app_sender%"
            to_email:   "%mailer_team_receiver%"
            subject:    "An Error Occurred!"
            level:      debug