You could consider using the Transaction Mail extension as this creates an Activity for all such outgoing mail
since receipts are presumably going out because you have selected a civi Contribution page that has Receipting turned on why not BCC yourself on that
From UI you can set the contact email to 'On-hold' this will prevent contact from getting any automated email from CiviCRM. The other option is use to mailing hooks to abort emails being sent based on some flag set for the client.
This issue is recorded here:
And there is currently some work being done on this here:
If you are keen to get this merged into core recommend testing it and commenting on the PR.
With an email activity you send an email to a non-primary address by going to the contact summary tab and hovering over the other email address and then clicking. But I can't see a more systematic way of sending to another category of address in CiviMail. Seems to rather defeat the purpose of having the different categories of address. Perhaps some specific ...
Take a look at Contributions -> Manage Contribution Pages, choose the relevant page, and then click on Configure, all the way over on the right. Then take a look at Receipt for that page,
might be at
and the e-mail addresses that a receipt is cc'd and bcc'd to.