As stated in the Symfony Docs it is possible, to add a failover configuration for mail servers:
Symfony's mailer supports high availability via a technique called "failover" to ensure that emails are sent even if one mailer server fails.
A failover transport is configured with two or more transports and the failover keyword:
MAILER_DSN="failover(postmark+api://ID@default sendgrid+smtp://KEY@default)"The failover-transport starts using the first transport and if it fails, it will retry the same delivery with the next transports until one of them succeeds (or until all of them fail).
Is it possible to use and configure this feature in TYPO3? If so, how?
Can other configurations like TLS Peer Verification (Symfony Docs) of the Symfony mailer be configured in TYPO3?
By default, SMTP transports perform TLS peer verification. This behavior is configurable with the verify_peer option. Although it's not recommended to disable this verification for security reasons, it can be useful while developing the application or when using a self-signed certificate:
$dsn = 'smtp://user:pass@smtp.example.com?verify_peer=0';
