Error When 'System Email Address' set via environment variable

Description

Emails do not send when the System Email Address is set via an environment variable because Swiftmailer appears to try to validate the true setting value before the environment variable gets evaluated.

2021-03-04 07:53:28 [-][-][o8q3812r4fbtucra6c85hhm1fb][error][Swift_RfcComplianceException] Swift_RfcComplianceException: Address in mailbox given [$CRAFTENV_SYSTEM_EMAIL_ADDRESS] does not comply with RFC 2822, 3.6.2. in /home/site/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Mime/Headers/MailboxHeader.php:355
Stack trace:
#0 /home/site/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Mime/Headers/MailboxHeader.php(272): Swift_Mime_Headers_MailboxHeader->assertValidAddress('$CRAFTENV_SYSTE...')
#1 /home/site/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Mime/Headers/MailboxHeader.php(117): Swift_Mime_Headers_MailboxHeader->normalizeMailboxes(Array)
#2 /home/site/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Mime/Headers/MailboxHeader.php(74): Swift_Mime_Headers_MailboxHeader->setNameAddresses(Array)
#3 /home/site/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Mime/SimpleMimeEntity.php(600): Swift_Mime_Headers_MailboxHeader->setFieldBodyModel(Array)
#4 /home/site/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Mime/SimpleMessage.php(213): Swift_Mime_SimpleMimeEntity->setHeaderFieldModel('From', Array)
#5 /home/site/vendor/yiisoft/yii2-swiftmailer/src/Message.php(103): Swift_Mime_SimpleMessage->setFrom(Array)

Steps to reproduce

1. Define an environment variable - $CRAFTENV_SYSTEM_EMAIL_ADDRESS. (NOTE: We use the .env.php method of setting environment variables which seems to work for all other site settings where we use environment variables.)
2. Set the System Email Address setting to that environment variable in the control panel.
3. Attempt to send an email via PHP module/plugin using the following code to initialize the Message object and the "from" header.

$this->emailSettings = Craft::$app->systemSettings->getSettings('email');
...
$message = new Message();
$message->setFrom([$this->emailSettings['fromEmail'] => $this->emailSettings['fromName']]);

Perhaps there is another way to load system settings with the environment variables already evaluated?

Additional info

• Craft version: 3.6.6
• PHP version: 7.3.27
• Database driver & version: MySQL 5.7.33

Start by tracing the email settings value from Craft::$app->systemSettings->getSettings('email') into yiisoft/yii2-swiftmailer/src/Message.php and the SwiftMailer mailbox validation shown in the stack trace. Reproduce the failure with an environment-variable System Email Address, then verify that the value is evaluated before SwiftMailer validates the From address and that sending succeeds.