Search operators you can use with Gmail – Gmail Help https://support.google.com/mail/answer/7190?hl=en
How to Search Mail in Gmail With Search Operators https://www.lifewire.com/how-to-search-mail-in-gmail-1172081
How to Send Email in WordPress using the Gmail SMTP Server http://www.wpbeginner.com/plugins/how-to-send-email-in-wordpress-using-the-gmail-smtp-server/
Configure Postfix to Use Gmail SMTP on Ubuntu – EasyEngine https://easyengine.io/tutorials/linux/ubuntu-postfix-gmail-smtp/