A user has "collaborate" permission on SharePoint Library; through the web UI the user can upload documents, but when using the Windows Explorer view or a mapped network drive led to
Access Denied error.
Thinking it could possibly be a network restriction, I made a packet capture and can confirm no port other than 80 is used to upload the documents.
So, what could be happening? Does she need to have "Full Control" permission in order to upload through a network drive or Windows Explorer?