Symptom:  You attempted to send an email and received a bounce message with a bounce reason similar to:

Delivery Failure : 550: Remote host said Sender Authorization check failed

Solution: This message indicates that recipients mail server rejected the sending mail server due to it failing an SPF check. 

More specifically, the sender's domain (example: sender@sendersdomain.com) has published an SPF record in their DNS.  An SPF record lists all servers allowed to send mail for sendersdomain.com, in an effort to prevent spoofing (i.e., a spammer sending mail pretending to be someone@sendersdomain.com).  A recipient's mail server can check the IP of the sending server versus the published list of approved sending servers in the sender's SPF record... and if the sending mail server is not in that list, the recipient's mail server may choose to reject the mail on the basis it came from a server not authorized to send mail for sendersdomain.com.

In short, the domain you sent the mail from (sendersdomain.com, in sender@sendersdomain.com), has published an SPF record, and the server you tried to send your mail through is not listed in that SPF record as approved to send mail for sendersdomain.com.

More details on SPF records can be found here:
http://knowledge.3essentials.com/web-hosting/article/521/Adding-an-SPF-record-for-your-domain.html