The mail is sent but there is an error

Hello, when sending mass e-mails from the admin side, it goes through but a warning message comes up. A welcome e-mail is not sent to new customers either. The screenshot of the error message is attached.

OC 3.0.3.8
PHP 7.4
MAIL SMTP



The following is what is written in the 29th line indicated:
$header .= 'Reply-To: =?UTF-8?B?' . base64_encode($this->reply_to_sender) . '?=' . ' <' . $this->reply_to . '>' . PHP_EOL;


Thank you to everyone who is interested.

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