All 7 Email Sending in PHP (PHPMailer & SMTP) questions
More in Email Sending in PHP (PHPMailer & SMTP)
How does PHP's built in mail function work, and why is it generally not recommended for production applications? What is PHPMailer, and what advantages does it provide over PHP's built in mail function for sending emails reliably? How do SPF, DKIM, and DMARC email authentication records help improve email deliverability and prevent your domain from being used for email spoofing? How do you send an HTML formatted email with an embedded image or attachment using PHPMailer? How should an application handle email sending failures gracefully, and why is sending emails asynchronously through a queue often preferred over sending them directly during a web request? What security considerations should be addressed when building an application feature that sends emails based on user input, such as a contact form or password reset flow?
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PHP › Email Sending in PHP (PHPMailer & SMTP)

What is SMTP authentication, and why is properly configuring it important for ensuring email deliverability?

Intermediate

Answer

SMTP authentication requires your application to provide valid credentials, typically a username and password, when connecting to an SMTP server to send an email, proving to the receiving mail server that the email genuinely originates from an authorized source rather than being potentially spoofed, and properly configuring this authentication, along with using an appropriate secure connection method, significantly improves the likelihood that your emails are trusted and delivered rather than rejected or marked as spam.
$mail->isSMTP();
$mail->Host = 'smtp.gmail.com';
$mail->SMTPAuth = true;
$mail->Username = 'you@example.com';
$mail->Password = 'app_specific_password';
$mail->SMTPSecure = 'tls';
$mail->Port = 587;
Real-world example A company configures its application to authenticate with its email provider's SMTP server using a dedicated application specific password rather than the account's actual login password, following the provider's recommended security practice for automated sending.

Common follow-ups: What is the difference between SSL and TLS options for SMTP connections?;Why should application specific passwords be used instead of a real account password?

Security;RESTful API Development with PHP
All 7 Email Sending in PHP (PHPMailer & SMTP) questions