By default OpenCart 2 uses the standard mail PHP function to send emails.
To improve your emails delivery you might consider setting up your store to use SMTP for all outgoing mail settings.
Additionally, the SMTP option allows you to use not only your own mail SMTP server but an external mail service
To configure OpenCart 2 SMTP settings, please login your admin Dashboard and refer to the settings section from the main navigation menu.
From the Stores List click the edit button and refer to the Mail tab.
In case you would like to use the standard PHP mail function you do not need to configure the SMTP settings on this page. In case you would like to set an additional email that will receive all notifications from your store you can set it in the Additional Alert E-Mail field. If you need to have several emails configured, please separate the email addresses with commas.