I am showing these errors in my system log while requesting a password reset, and the customer don't receive the email.

[2020-01-09 05:16:28] main.CRITICAL: Exception message: Invalid header value detected
Trace: <pre>#1 Zend\Mail\Header\AbstractAddressList->getFieldValue(true) called at [vendor/zendframework/zend-mail/src/Header/AbstractAddressList.php:209]
#2 Zend\Mail\Header\AbstractAddressList->toString() called at [vendor/zendframework/zend-mail/src/Headers.php:426]
#3 Zend\Mail\Headers->toString() called at [vendor/zendframework/zend-mail/src/Message.php:546]
#4 Zend\Mail\Message->toString() called at [vendor/magento/framework/Mail/EmailMessage.php:217]
#5 Magento\Framework\Mail\EmailMessage->toString() called at [vendor/magento/framework/Mail/EmailMessage.php:209]

This is a bug of Magento version 2.3.3. It does not allow the use of special characters such as ã, ơ in-store email address or sender mail name or etc...

This is a changed path in Magento


Please replace function with this code in above file :

public function toString(): string {
    return $this->message->toString();

Yes, it is a bug with encoding ...

In the EmailMessage class (vendor/magento/framework/Mail/EmailMessage.php) the encoding can be set if passed in an argument in the constructor, but it is null by default.

if ($encoding) {

So, I think the best practice use type in di.xml in your module without rewrite and plugins

<type name="Magento\Framework\Mail\EmailMessage">
       <argument name="encoding" xsi:type="string">utf-8</argument>

Here is the class in case some one wonders what is going on:

class SenderBuilder extends \Magento\Sales\Model\Order\Email\SenderBuilder
     * @throws \Magento\Framework\Exception\LocalizedException
     * @throws \Magento\Framework\Exception\MailException
    public function send()
        //Removed Customer name because of diacritics

        $copyTo = $this->identityContainer->getEmailCopyTo();
        if (!empty($copyTo) && $this->identityContainer->getCopyMethod() == 'bcc') {
            foreach ($copyTo as $email) {

        $transport = $this->transportBuilder->getTransport();

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