I was able to send mails with PHPMailer.
<?php use PHPMailer\PHPMailer\PHPMailer; use PHPMailer\PHPMailer\Exception; require 'vendor/autoload.php'; $sender = getenv('CLOUDRON_MAIL_FROM'); $senderName = 'LAMP Test Email'; $recipient = 'test@cloudron.io'; $usernameSmtp = getenv('CLOUDRON_MAIL_SMTP_USERNAME'); $passwordSmtp = getenv('CLOUDRON_MAIL_SMTP_PASSWORD'); $host = getenv('CLOUDRON_MAIL_SMTP_SERVER'); $port = (int) getenv('CLOUDRON_MAIL_SMTP_PORT'); $subject = 'Cloudron test (SMTP interface accessed using PHP)'; $bodyText = "Email Test\r\nThis email was sent through the Cloudron SMTP interface using the PHPMailer class."; $bodyHtml = '<h1>Email Test</h1>'; $mail = new PHPMailer(true); try { $mail->isSMTP(); $mail->setFrom($sender, $senderName); $mail->Username = $usernameSmtp; $mail->Password = $passwordSmtp; $mail->Host = $host; $mail->Port = $port; $mail->SMTPAuth = true; $mail->addAddress($recipient); $mail->isHTML(true); $mail->Subject = $subject; $mail->Body = $bodyHtml; $mail->AltBody = $bodyText; $mail->Send(); echo "Email sent!" , PHP_EOL; } catch (phpmailerException $e) { echo "An error occurred. {$e->errorMessage()}", PHP_EOL; //Catch errors from PHPMailer. } catch (Exception $e) { echo "Email not sent. {$mail->ErrorInfo}", PHP_EOL; } ?>(Adapted from https://docs.aws.amazon.com/ses/latest/DeveloperGuide/send-using-smtp-php.html )