このエラーメッセージに関する原因と対処に関して説明します。
エラーメッセージ(英語)：
wordpress not sending email
対処
関連リンク
Wordpress not sending email via sendmail - Stack Overflow
https://stackoverflow.com/questions/41025238/wordpress-not-sending-email-via-sendmail
How to Use SMTP Server to Send WordPress Emails - WPBeginner
https://www.wpbeginner.com/wp-tutorials/how-to-use-smtp-server-to-send-wordpress-emails/
Connect WP to Send System Email via Amazon SES | Amazon Web ...
https://www.freelancer.com/projects/wordpress/connect-send-system-email-via/
A Simple Solution for WordPress Not Sending Email - Ninja Forms
https://ninjaforms.com/ninja-forms-email-mailgun/
Fix wordpress not sending emails issue by Shahidbahader - Fiverr
https://www.fiverr.com/shahidbahader/fix-wordpress-not-sending-emails-issue
WP Email Delivery - Managed Email Delivery For WordPress
https://www.wpemaildelivery.com/
blog - WP Email Delivery
https://www.wpemaildelivery.com/blog/
WP Mail SMTP by WPForms | WordPress.org
https://wordpress.org/plugins/wp-mail-smtp/
Use SendGrid with Thrive Leads for WordPress - YouTube
https://www.youtube.com/watch?v=DpVWupY_YR0
https://www.youtube.com/watch?v=DpVWupY_YR0