anti spam systems use black lists these are lists of server that have been reported as sources of spam.

These lists reset after 24-48 hours so the simplest practice is just to wait a day and try resending.

However if the problem is urgent you can check some common blacklists here, to use this you will need to know the name of your send mail server this can be found in your email programs outgoing or SMTP mail settings e.g. for a bigpond customer this would be mail.bipond.com

The test will run a check against 100+ servers to see if you/they are being blacklisted if you are on a blacklist there is a procedure to get removed just gop the list servers web site and request a de-listing

I hope this helps.