Were you using a link in your email letter, if so it might be on the RBL list. You can check spamhaus to see if your mail server IP has been blocked, a lot of ISP's are now using there database to fight against spam. Were you using the mail server of your ISP? Hosting company? Third party SMTP? Did you send yourself an email to an email account that has spam assassin installed on it to check the spam assassin score? Anything above 5 will trigger spam assassin and your mail will not go out.