5.7.3 Authentication unsuccessful

Hi,

2 weeks ago, email from our website stopped being sent out and began getting “5.7.3 Authentication unsuccessful” in the email logs.

My knowledge of most of this type of stuff is very limited and I have no idea what could be causing it.

After running the diagnostic test I get the following:

OS: Linux fairways-website 4.4.0-36-generic #55-Ubuntu SMP Thu Aug 11 18:01:55 UTC 2016 x86_64
PHP: Linux 5.6.25-2+deb.sury.org~xenial+1 C
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=Yes, allow_url_fopen=Yes, mcrypt=No, zlib_encode=Yes
WordPress: 4.6.1 en_US UTF-8
WordPress Theme: Fairways
WordPress Plugins: Advanced Custom Fields: Repeater Field, AddToAny Share Buttons, Advanced Custom Fields, Contact Form 7 Modules: Hidden Fields, Contact Form 7, Duplicate Post, Postman SMTP, Related Posts By Taxonomy, Smartlook Visitor Screen Recording, Yoast SEO, WP Migrate DB, WP ULike
WordPress wp_mail Filter(s): wp_staticize_emoji_for_email
WordPress phpmailer_init Action(s): wpcf7_phpmailer_init
Postman: 1.7.2
Postman Sender Domain (Envelope|Message): zudu.co.uk | zudu.co.uk
Postman Prevent Message Sender Override (Email|Name): No | No
Postman Active Transport: SMTP (smtp:tls:login://smtp.office365.com:587)
Postman Active Transport Status (Ready|Connected): Yes | Yes
Postman Deliveries (Success|Fail): 62 | 63
Postman File Locking (Enabled|Temp Dir): No | /tmp

The emails were working fine before 2 weeks ago but have suddenly stopped and we cant think of what could have caused it. Does anyone have any idea what could be happening here?

I’m using Contact Form 7 on the site and Digital Ocean for hosting