How to fix Internal Server Error #8 when sending mail via yii\swiftmailer\Mailer in Yii2?

Question

Yii

When trying to send a letter via yii\swiftmailer\Mailer, the server gives an error:

Error (#8)
An internal server error has occurred.

Moreover, this code is guaranteed to work on the test server. Tell me which way to dig?
PPH version used is 7.0.1
What extensions does SwiftMailer depend on?

Answer 1

The question is no longer relevant. The problem was in the code.

Answer 2

Copy your message submission code here.

Answer 3

what was the problem?