535 Incorrect authentication data

No longer recieving email notifacations from form entries on the site as of 13:50GMT on the 7th Feb last week with the status showing as 535 Incorrect authentication data. I have recently took over responsibility of the site so have not changed anything to cause this to my knowledge, any suggestions or help would be greatly appreciated.

Diagnostic Test:

Mailer: postsmtp
HostName: prodisplay.com
cURL Version: 7.67.0
OpenSSL Version: OpenSSL/1.1.1d
OS: Linux prodisplay.servers.prgn.misp.co.uk 3.10.0-1062.4.1.el7.x86_64 #1 SMP Fri Oct 18 17:15:30 UTC 2019 x86_64
PHP: Linux 7.3.14 C
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=Yes, allow_url_fopen=Yes, mcrypt=No, zlib_encode=Yes
WordPress: 5.3.2 en_US UTF-8
WordPress Theme: Pro Display
WordPress Plugins: Gravity Forms, Add Expires Headers Plugin, Advanced Custom Fields PRO, All-in-One WP Migration, Activity Log, Attachment Pages Redirect, Breadcrumb NavXT, Classic Editor, Contact Form 7, Cookie Notice, Custom Twitter Feeds, Elementor, Essential Addons for Elementor, Fast Velocity Minify, Multiple Columns for Gravity Forms, Gravity Forms – Placeholders add-on, Gravity Forms Debug Add-On, Hotjar, Leverage Browser Caching, Lightweight Grid Columns, Logo Slider WP, Multi Image Metabox, New RoyalSlider, PHP Compatibility Checker, Post Duplicator, Post SMTP, Post Types Order, Real 3D Flipbook, Really Simple SSL, Regenerate Thumbnails, Shortcoder, Shortcodes Ultimate, TablePress, Category Order and Taxonomy Terms Order, UberMenu – Sticky Menu Extension, UberMenu 2 – WordPress Mega Menu Plugin, Use Google Libraries, Wordfence Security, Yoast SEO Premium, WP Fastest Cache, WP Gallery Custom Links, Smush
WordPress wp_mail Owner: /home/prodisplay/public_html/wp-content/plugins/post-smtp/Postman/PostmanWpMailBinder.php
WordPress wp_mail Filter(s): wp_staticize_emoji_for_email, PostsmtpMailer->get_mail_args
WordPress wp_mail_from Filter(s): wordfence::fixWPMailFromAddress
WordPress phpmailer_init Action(s): wpcf7_phpmailer_init, PostsmtpMailer->phpmailer_smtp_init
Postman: 2.0.10
Postman Sender Domain (Envelope|Message): prodisplay.com | prodisplay.com
Postman Prevent Message Sender Override (Email|Name): Yes | Yes
Postman Active Transport: SMTP (smtp:tls:plain://prodisplay.servers.prgn.misp.co.uk:587)
Postman Active Transport Status (Ready|Connected): Yes | Yes
Postman Deliveries (Success|Fail): 46563 | 1489