The answer to the following KB question lists some limitations on the S3Uploader (and I would assume the other S3* transformers):
https://knowledge.safe.com/questions/31838/s3uploader-and-network-proxy.html
Specifically, the limitations are listed as:
...there are some non-supported configurations:
- Custom proxy maps
- NTLM or Digest proxy authentication (as seen when you go to Tools-->FME Options-->Network Proxy-->'Use Authentication')
It would be great to see the S3* suite of transformers updated to honour the full set of proxy configurations. We are specifically interested in NTLM authentication, but why not bring it inline with the rest of the FME software and support everything?