I have an scenario where external vendor will send a csv file as attachment in the email which PI needs to pick it up and send it via SFTP to another vendor.
There is no mapping required as only SFTP and PGP encryption is done.
It will be simple file transfer to end client.
From design perspective, is this possible and any scenario examples is appreciated.
Your scenario sounds very much possible. You might find these two blogs from Shabarish useful for the SFTP/PGP receiver part:
Also, search SCN for examples of mail to file pass-through scenarios, as this was already widely discussed.
I believe PayloadSwapBean is what you are looking for
SAP Help for it:
And Michal's blog on how to use it in practice with sender mail adapter: