The email log

Mail > Email log has a row for every email the site sends: date, recipients, subject, result and the plugin that sent it. Click the subject to see the error and the connection used.

• Sent: the SMTP server accepted the email. From there it is up to the recipient's provider: if it is not in the inbox, look in spam.
• Failed: the email did not leave. The error and a hint are under the subject.
• Other plugin: another plugin sent the email in place of WordPress, so Mail cannot know the result.

The free version keeps 30 days and deletes older rows once a day. With Pro you can keep up to a year, switch on Also keep the content of the emails to read and resend them, search the whole log and export it to CSV. The content can include personal data: keep it only if you need it, and mention it in your privacy policy.