When email fails a 550 error may occur. In most cases the email has been blocked as spam.

• 550 5.7.1: Service unavailable: client [###] blocked using ### (Exchange Server)
• 550 5.7.1: Message rejected as spam by Content Filtering (Exchange Server)
• 550 This message was classified as SPAM and may not be delivered
• 550 High probability of spam (Gmail)
• 550 5 2 1 mail from refused spam site
• 550 Your message has been rejected because you have been detected sending spam (Sending from Rackspace)
• 550 Message contained unsafe content