How to hook wp_mail to add a custom email as BCC for each sent email?

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You have multiple options:

  1. The function is pluggable (i.e., you could write up your own wp_mail function and make it do what you’d like).
  2. There’s the phpmailer_init hook, which gives you access to the $phpmailer object (and thus lets you AddBcc another email address.
  3. There’s the wp_mail hook, which gives you (amongst other things) access to $headers, where you could add another email address to the 'bcc' field.