I tried to disable all plugin and run again.
Mailer: phpmailer
HostName: walkingbook.tw
cURL Version: 7.78.0
OpenSSL Version: OpenSSL/1.1.1k
OS: Linux gator4125.hostgator.com 4.19.150-76.ELK.el6.x86_64 #1 SMP Wed Oct 7 01:34:10 CDT 2020 x86_64
PHP: Linux 7.3.29 C
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=Yes, allow_url_fopen=Yes, mcrypt=No, zlib_encode=Yes
WordPress: 5.8 zh_TW UTF-8
WordPress Theme: Twenty Twenty-One
WordPress Plugins: Post SMTP
WordPress wp_mail Owner: /home4/hanktsai/public_html/wp-config.php
WordPress wp_mail Filter(s): wp_staticize_emoji_for_email, PostsmtpMailer->get_mail_args
WordPress phpmailer_init Action(s): PostsmtpMailer->phpmailer_smtp_init
Postman: 2.0.23
Postman Sender Domain (Envelope|Message): mail.walkingbook.tw | mail.walkingbook.tw
Postman Prevent Message Sender Override (Email|Name): Yes | Yes
Postman Active Transport: SMTP (smtp:tls:plain://mail.walkingbook.tw:587)
Postman Active Transport Status (Ready|Connected): Yes | Yes
Postman Deliveries (Success|Fail): 0 | 0