Not sending a notification email to admin when someone registered

Hello, I use internal PHP mail instead of SMTP for the WordPress app because I have no problem sending emails through PHP mail by having DKIM, DMARC, etc pass the security. However, my WordPress gives the following error message whenever your plugin trying to send a notification email to me when there is someone registered. It looks like as follows:
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Mailer:
Default (none) PHPMailer was able to connect to SMTP server but failed while trying to send an email.

Source:
… /wp-content/plugins/apply-online/public/class-applyonline-public.php (line: 1008)
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I used WP Mail SMTP’s debug events feature to find this error related to Apply Online plugin. Other plugins like “WP Mail SMTP” and “WP Cerber”, they have no problem sending emails to me via PHP mail.

Thank you very much if you can help me solve the problem.