When receiving forwarded files, the replacement permissions for the sender apply, not the replacement permissions for the originating user.
When a forwarded file is received that duplicates a file already in the Received mailbox, the replacement privileges of the sender of the file, not the originating user, are used to determine whether to reject the incoming file or overwrite the existing file.
For instance, suppose Adam forwards files to Charles via Beth. Charles allows Adam to replace files but does not allow Beth to replace files. If Adam forwards Charles a file that is already in the received mailbox, Charles will reject the file, because he is receiving it from Beth, who is not permitted to replace files.
In the same way, if Charles gives Beth permission to replace files but not Adam, duplicate files sent to Charles from Adam will still be overwritten if they are forwarded through Beth, who does have replacement permissions.