Why are messages I send being rejected with a high spam score?

Some users on UWinGmail are having messages that they send returned to them with an error stating that the message has a high spam score:

550 This message was rejected for policy reasons (high spam score).

This problem usually occurs for users who are sending mail using their mobile phone. The cause of the problem is that the IP address currently in use by the mobile phone has been placed on a Spam Blacklist for sending spam messages. This is most frequently the case where the previous user who had that IP address was detected sending spam.

At this time there is no fix.  The Spam Blacklist will eventually remove the IP address from the blacklist automatically and mail messages sent from that IP address will then be accepted for delivery.

A workaround is to send the message using the UWinGmail web interface instead of the mobile phone.