Unable to forward phishing emails

Sometimes when I try to forward a phishing scam email to the organization being spoofed, Cox rejects the forwarded message. The Cox server returns an error message like:

server message: ... This message was undeliverable. This message has been found to be a +potential spam message, and has therefore been blocked. Please visit ww2.cox.com/coxagainstspam for more information.

This does not occur when I forward to phishingreport@cox.net or abuse@cox.net, but it does occur with other phishing-report addresses, such as:

spoof@americanexpress.com (American Express)
phishalert@navyfederal.org (Navy Federal Credit Union)
reportphish@wellsfargo.com (Wells Fargo)

My mail setup: Linux system, MUA is Mutt v1.10, MTA is msmtp v1.8 (a simplified Sendmail variant). The outgoing mail server is smtp.cox.net, I use TLS and port 587. I am usually able to send and receive other emails without getting these errors.

This problem has been reported several times in the forum, e.g.:

forums.cox.com/.../cox-blocked-efforts-to-report-phishing-e-mail
forums.cox.com/.../forwarding-phishing-e-mails
forums.cox.com/.../phishing-emails
forums.cox.com/.../72618
forums.cox.com/.../spam-phishing
forums.cox.com/.../forward-phishing-email-to-support

but, thus far no solution I could find.

Is there a way to bypass the spam filter when forwarding a phishing email to a phishing-report address other than Cox's?