Unfortunately, it is very common for forwarding to cause blacklisting with many large email providers. This most commonly occurs when large amounts of mail are forwarded and then marked as spam after it has been received in the end user's inbox. This results in your server's sending reputation being penalized since it was the last server to send the mail. Ultimately, your server will become blacklisted with the email provider and it will affect all of the users on the server.
A solution to this is to stop the forward altogether. Using a mail fetcher, you can access mail in a remote system without having the mail routed through that system's server. This way, you can mark the message as spam without it affecting your email server's reputation.