invalid_grant error message

Hi there, I hope you can help me.

I have started getting this error message in the past couple of weeks, and no emails are sending (after it was working perfectly).

Nothing changed on the site, but all of a sudden each email in the log says:

Bad Request(invalid_grant)

I then try to send test email and get the same response.

Please can you help me?

Some diagnosstic information as follows:
PHP: Linux 5.6.25-1+deb.sury.org~precise+1 C
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=Yes, allow_url_fopen=Yes, mcrypt=Yes, zlib_encode=Yes
WordPress: 4.5.3 en_US UTF-8
WordPress Plugins: ManageWP – Worker, Gravity Forms, Advanced Custom Fields Repeater Collapser, Advanced Custom Fields PRO, Akismet, iThemes Boom Bar, Column Shortcodes, Custom Recent Posts Widget, Email Log, Event Organiser, File Manager, GG Widget Title Link, Gravity Forms Logging Add-On, Gravity Forms MailChimp Add-On, Image Rotation Fixer, iThemes Sync, Jetpack by WordPress.com, Manage Notification E-mails, Pin It Button for Pinterest, Postman SMTP, Redirection, Regenerate Thumbnails, SeedProd Coming Soon Pro, Simple CSS for widgets, Soliloquy – Instagram Addon, Soliloquy, Stream, User Switching, White Label Branding for WordPress, WooCommerce Branding, WooCommerce Product Add-ons, WooCommerce, WooSidebars, WooCommerce Helper, WordPress HTTPS, WP-PageNavi
WordPress wp_mail Filter(s): wp_staticize_emoji_for_email, EmailLog->log_email, wlb_branding->filter_wp_mail
Postman: 1.7.2
Postman Prevent Message Sender Override (Email|Name): Yes | Yes
Postman Active Transport Status (Ready|Connected): Yes | Yes
Postman Deliveries (Success|Fail): 340 | 28

Best wishes,
Andrew

https://wordpress.org/plugins/postman-smtp/