The SMTP settings for Gmail are quite specific.
You should use Gmail SMTP setting if you also use a Gmail account as your main site email, whether this is using your own domain though Gmail especially if you are using myname@Gmail.com as your main site email. Doing this will prevent your customers getting “email may not have originated from the specified domain” errors when they receive email from your cart.
First login to your OpenCart admin
Go system->settings in the menu
Click on the mail tab
SMTP Username: (the full email you use to login to your Gmail, i.e myname@Gmail.com)
SMTP Username:(the password you use to login to Gmail)
Set New Order Alert Mail or New Order Alert Mail depending on if you want alerts sent to yourself from your OpenCart on order/accounts being setup
Add Additional Alert E-Mails if you want alert emails also sent to another of your own emails.
Example of settings:
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