Finally here is diagnostic info. Thanks for your help, eager to get this resolved.
HostName: redacted.com
OS: Linux red.redacted.com 3.10.0-714.10.2.lve1.5.9.el7.x86_64 #1 SMP Thu Jan 11 14:17:06 EST 2018 x86_64
PHP: Linux 5.6.35 en_US.UTF-8
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=Yes, allow_url_fopen=Yes, mcrypt=Yes, zlib_encode=Yes
WordPress: 4.9.6 en_US UTF-8
WordPress Theme: Custom Theme
WordPress Plugins: Gravity Forms, a3 Lazy Load, Autoptimize, iThemes Security, Disable Comments, Duplicate Post, Duplicator, Google Analytics Dashboard for WP (GADWP), Anti-Malware Security and Brute-Force Firewall, Members, Menu Social Icons, Post SMTP, Redirection, Responsive Lightbox & Gallery, Sassy Social Share, Shortcodes Ultimate, WooSlider, Yoast SEO, Smush, WP-Sweep, NextGEN Gallery
WordPress wp_mail Filter(s): wp_staticize_emoji_for_email
Postman: 1.8.9
Postman Sender Domain (Envelope|Message): redacted.com | redacted.com
Postman Prevent Message Sender Override (Email|Name): No | No
Postman Active Transport: SendGrid API (https://api.sendgrid.com:443)
Postman Active Transport Status (Ready|Connected): Yes | Yes
Postman Deliveries (Success|Fail): 20427 | 31
Postman PHP LogLevel: 30000