A number of innocent-looking words have been banned because they have been used in multiple spam messages, and it was the only way to bring them to a stop. If you "Search" past messages (see "Search" top of list) you will find numerous messages describing how to get around this.

The most common reason a message is rejected is when a URL has been used, because ht tp has been banned (without the space). Either delete the ht tp, because the website will come up just the same, or put a space in it as I have done.

Also put a space in words like se x.
If you can't find which word has been banned try deleting part of your message and see whether it gets though when you "Preview Message" - and keep deleting parts and previewing them until the "naughty" word shows up.

Sue