I configured postfix to allow relay for sasl authenticated users, so I can use my own postfix server to send mails from my phone. Now, when I send an email from my phone, I would also like to forward this mail to an other address (so I don't have to sync my sent items from the phone to the desktop). What I want to accomplish is something like adding a bcc to a relayed mail. Is this possible?
There's a feature for that, but I've not seen how to do it for emails from your phone only - it's triggered by any email with a given envelope sender address. At most you can constrain it with an IP range so that mails sent from your desktop's IP address don't trigger it.
The feature you're looking for is
sender_bcc_maps
which should be a table. See the man page for postconf(5).