While I appreciate this is an old thread. I didn't catch this post the first time around. Unfortunately, the OP has not revisited this.
I'm afraid the answer as given was incorrect. I recognize the error message in question and although the OP did not explicitly say so - I can state with 100% confidence that
1. They were using the webmail platform. 2. This wasn't a bounce message, this is an actual error message put on the screen when he tried forwarding the mail using the webmail platform.
The outbound SMTP server parses the mail and if it deems the content is spam, refuses to send it. So the webmail platform instantly reports the error message back to the user.
I'm all for stopping spam, but I've always argued that your spam filtering is too sensitive. In addition, many organizations such as banks, ask that you forward suspected phishing emails as an attachment to them. Something which this outbound spam filtering prevents, in some cases despite having not filtered the mail as it comes in.
Because email clients don't use the same SMTP servers as the webmail platform, they don't suffer this problem. Which is why @rjerinic can send using his iPhone and Outlook.
Please go and have words with Liberty Global - and tell them it's about time they fixed this. They are hurting their webmail users with overzealous outbound filtering.
Only use Helpful answer if your problems been solved.