Add an email recipient to an email sent from Salesforce

Question

Is it possible to dynamically add an email recipient to an email sent from Salesforce? What I wanted to achieve is:
If an email is sent to the email address [email protected] it should additionally be sent to the email address [email protected].
On the other way around, if an email is sent to [email protected], it should be sent to [email protected] too.

I don't want to do this from a specific quick action, lightning component or visualforce page. I want to do this every time an email will be sent from Salesforce. Regardless of the function that is sending the email.
I already tried to put additional email addresses into the EmailMessage.ToAddress field with a before insert trigger but this does not seem to work.
Any ideas if I can get this done?

Answer 1

If you generate your emails from APEX, you can do this while generating the email. However, what you are asking for, I do not believe is possible. There is no "catch-all" for outbound emails from SFDC. (Note: the before-insert trigger on the email object would fire after the email has already been sent, but before committing to the system.)

Here is a related question/answer: Is it possible to modify outgoing emails via Apex?