Email Delivery Problem

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

HostName: ip-146-66-105-158.siteground.com
OS: Linux sgp72.siteground.asia 3.12.18-clouder0 #1 SMP Wed Mar 28 14:22:08 EEST 2018 x86_64
PHP: Linux 5.6.39 C
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=Yes, allow_url_fopen=Yes, mcrypt=Yes, zlib_encode=Yes
WordPress: 5.0.2 en_US UTF-8
WordPress Theme: Hestia
WordPress Plugins: Polylang, FooGallery, Classic Editor, Ninja Forms styler for Elementor, Elementor, Essential Addons for Elementor, Font Awesome 4 Menus, FooBox Image Lightbox, Google Analytics for WordPress by MonsterInsights, Google Language Translator, Header, Footer & Blocks for Elementor, Jetpack by WordPress.com, JetSticky For Elementor, Kadence WooCommerce Elementor, Menu Image, Menu Social Icons, PopBox For Elementor, Nav Menu Images, Newsletter Subscription Form, Ninja Forms, Ninja Tables, Popup Maker, Post SMTP, PT Elementor Addons Lite, SG Optimizer, SiteOrigin Widgets Bundle, The Events Calendar, Social Media and Share Icons (Ultimate Social Media), Use Any Font, WidgetKit for Elementor, WooCommerce, WordPress Importer, Yoast SEO, WordPress Starter, WP EasyCart, WP Google Maps, WP Mega Menu
WordPress wp_mail Filter(s): wp_staticize_emoji_for_email
Postman: 1.9.5
Postman Sender Domain (Envelope|Message): gmail.com | gmail.com
Postman Prevent Message Sender Override (Email|Name): No | No
Postman Active Transport: SMTP (smtp:tls:oauth2://smtp.gmail.com:587)
Postman Active Transport Status (Ready|Connected): Yes | Yes
Postman Deliveries (Success|Fail): 0 | 27