There is no possibility to put a copy of an outgoing email to a specific folder. This would be useful for facilities with multiple agents working on incoming mail.

To solve this, you could either
(1) add a functionality to set up filter rules for outgoing mails too (place copy in folder X when rule applies)
or
(2) add an identity setting "automatic BCC" so when agents write an email using a certain identity, a copy will be sent to a different account. This would not be as clean of a way as (1) (which resembles Thunderbid's "Copies & Folders" -> "When sending messages, automatically: Place a copy in" feature) but would allow for a workaround.