Random emails NOT being sent

My web site emails me when customers place orders.

Over the last few days I have had 2 emails not arrive to confirm the orders yet all other emails arrived.

Here is the diagnostic test.

HostName: ip-77-104-128-198.siteground.com
OS: Linux uk27.siteground.eu 3.12.18-clouder0 #20 SMP Mon Feb 26 12:15:53 EET 2018 x86_64
PHP: Linux 7.0.33 C
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=Yes, allow_url_fopen=Yes, mcrypt=Yes, zlib_encode=Yes
WordPress: 5.0.2 en_GB UTF-8
WordPress Theme: Clean Commerce Child
WordPress Plugins: AfterShip – WooCommerce Tracking, Akismet Anti-Spam, Autoptimize, ClickToAddress UK Postcode Lookup Integration, Contact Form 7, Jetpack by WordPress.com, Post SMTP, Really Simple CAPTCHA, Regenerate Thumbnails, Search Everything, SG Optimizer, WooCommerce Offline Credit Card Processing, WooCommerce PDF Invoices & Packing Slips, WooCommerce Sendinblue Newsletter Subscription, WooCommerce, Wordfence Security, WordPress Importer, WP Child Theme Generator
WordPress wp_mail_from Filter(s): wordfence::fixWPMailFromAddress
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_On_Hold_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
Postman: 1.9.5
Postman Prevent Message Sender Override (Email|Name): No | No
Postman Active Transport Status (Ready|Connected): Yes | Yes
Postman Deliveries (Success|Fail): 7146 | 11