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)：
cannot send mail
対処
関連リンク
Cannot send mail using Amazon SES: 554 Message rejected: Email ...
https://support.plesk.com/hc/en-us/articles/115004161894-Cannot-send-mail-using-Amazon-SES-554-Messa・・・
amazon ec2 - Unable to send email from a ec2 instance using ...
https://serverfault.com/questions/673793/unable-to-send-email-from-a-ec2-instance-using-postfix
Ubuntu 14.04 on Amazon EC2- postfix ...
https://serverfault.com/questions/805173/ubuntu-14-04-on-amazon-ec2-postfix-segmentation-fault
email - Postfix - can send mail to outside directly ...
https://serverfault.com/questions/829412/postfix-can-send-mail-to-outside-directly-from-server-but-n・・・
smtp - Using postfix to send email in redmine
https://serverfault.com/questions/735172/using-postfix-to-send-email-in-redmine
linux - How to make Ubuntu Server 12.04 can ...
https://serverfault.com/questions/532533/how-to-make-ubuntu-server-12-04-can-send-mail
amazon ec2 - Again: sending mail (through php) from an EC2 instance ...
https://stackoverflow.com/questions/12709239/again-sending-mail-through-php-from-an-ec2-instance/129・・・
What is the required configuration steps to have a Spring Boot ...
https://stackoverflow.com/questions/46245538/what-is-the-required-configuration-steps-to-have-a-spri・・・
Send email without Outlook or System.Net.Mail
https://stackoverflow.com/questions/19631500/send-email-without-outlook-or-system-net-mail
Cannot send mail with no "From" header ---- codeigniter 3
https://stackoverflow.com/questions/30047716/cannot-send-mail-with-no-from-header-codeigniter-3