Make sure you are not sending something that may look like contact information (part number that looks like a phone number, email address, anything that looks like a link).
Is the problem with one specific message or user? Make sure the user is still registered. Try going to the user's feedback or profile page, use the contact link there to send a new message. If prompted for an item number, choose "not about an item" if the original listing has expired. If you have more than one account, try sending a message using the other account.
There may be limits to the number of messages you can send, so if you have made many attempts, waiting until tomorrow is not the worst idea.
