400 BAD REQUEST ERROR

Hi, i’m new to WP so got a lot to learn. Would anyone be able to help with this error?

Error calling POST https://www.googleapis.com/gmail/v1/users/me/messages/send: (400) Bad Request

OS: Linux info 3.0 #1337 SMP Tue Jan 01 00:00:00 CEST 2000 all GNU/Linux
PHP: Linux 7.0.6 C
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=No, allow_url_fopen=Yes, mcrypt=Yes, zlib_encode=Yes
WordPress: 4.5.2 en_US UTF-8
WordPress Theme: Big Point
WordPress Plugins: Admin Menu Manager, Contact Form 7, Flamingo, Google Analytics Dashboard for WP, Lollum Framework, oAuth Twitter Feed for Developers, Postman SMTP, Regenerate Thumbnails, Revolution Slider, Custom Order Status Pro for WooCommerce, Woo Import Export, Advanced AJAX Product Filters for WooCommerce, WooCommerce Checkout Manager, WooCommerce Colors, WooCommerce Customizer, Booster Plus for WooCommerce, Booster for WooCommerce, WOOF – WooCommerce Products Filter, WooCommerce Xero Integration, WooCommerce, WP Maintenance Mode
WordPress wp_mail Filter(s): wp_staticize_emoji_for_email
WordPress phpmailer_init Action(s): wpcf7_phpmailer_init, WC_Email_New_Order->handle_multipart, WC_Email_Cancelled_Order->handle_multipart, WC_Email_Failed_Order->handle_multipart, WC_Email_Customer_Processing_Order->handle_multipart, WC_Email_Customer_Completed_Order->handle_multipart, WC_Email_Customer_Refunded_Order->handle_multipart, WC_Email_Customer_Invoice->handle_multipart, WC_Email_Customer_Note->handle_multipart, WC_Email_Customer_Reset_Password->handle_multipart, WC_Email_Customer_New_Account->handle_multipart, WC_Email_Adv_Statuses->handle_multipart, WC_Email_Adv_Statuses->handle_multipart, WC_Email_Adv_Statuses->handle_multipart, WC_Email_Adv_Statuses->handle_multipart, WC_Email_Adv_Statuses->handle_multipart, WC_Email_Adv_Statuses->handle_multipart, WC_Email_Adv_Statuses->handle_multipart, WC_Email_Adv_Statuses->handle_multipart, WC_Email_Adv_Statuses->handle_multipart, WC_Email_Adv_Statuses->handle_multipart, WC_Email_Adv_Statuses->handle_multipart
Postman: 1.7.2
Postman Sender Domain (Envelope|Message): back2bed.co.uk | back2bed.co.uk
Postman Prevent Message Sender Override (Email|Name): No | No
Postman Active Transport: Gmail API (https:oauth2://www.googleapis.com:443)
Postman Active Transport Status (Ready|Connected): Yes | Yes
Postman Deliveries (Success|Fail): 0 | 0

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