Email server: how do I filter messages sent by me?

[szatox]

Yet another topic I really didn't know where to post.

I'm building my email server. The first and most basic bits are in place: postfix (MTA), dovecot (mailboxes, sasl, imap), mysql (domains, auth info, aliases), even certbot. So far so good.
I configured postfix to BCC the sender if sender's address is found in the database.
Now I'm trying to figure out a way to filter those messages from dovecot's Inbox folder to Sent folder.
In the most basic case I can use a sieve like that:

[szatox]

For the record, I found extension to dovecot's sieve. I hoped for a more "native" solution, but it works well enough.
I came with a solution that:
- binds envelope TO and FROM to variables
- invokes an external script with bound variables as parameters
- binds output from script to another variable and uses error code to determine test result

The external script is basically an SQL query asking the database whether or not the addresses provided belong to the same user.
There is also another extension, external data source, however I'd rather have my database return a simple answer than dump the whole table into a dictionary so I can process it myself every time a message reaches some mailbox.