AT&T requires that all e-mail sent through its servers follows the AT&T E-mail Guidelines as well as any applicable federal, state, and local laws. Following the e-mail guidelines does not guarantee that your e-mail will be delivered, however it should decrease the chances of your e-mail being misidentified and blocked as unsolicited bulk e-mail or Spam.

AT&T E-mail Guidelines: Best Practices for E-mail Senders

AT&T provides the following requirements/guidelines to assist in the proper delivery of e-mail through the AT&T Mail Servers. If these requirements are not met, your messages may not be accepted.

• Ensure that your e-mail follows all appropriate Requests for Comments (RFCs).
  http://www.rfc-editor.org
• Ensure that all e-mail complies with the Federal Can Spam Act of 2003. http://www.spamlaws.com/federal/can-spam.shtml
• Deliver e-mail through your ISPs mail servers, if available. Connecting directly from a dial-up or other dynamically assigned IP address is possible but may result in your mail not being delivered.
• Separate large quantities of e-mail into sections and deliver at periodic intervals.
• Send messages only from IP addresses that do not appear on any Real-time Blackhole list (RBL). Connections and messages from an IP address listed under a reputable RBL may not be accepted. To have your IP address removed from an RBL, contact the list’s manager directly.
• Do not send e-mail from an IP address where the sender’s identity changes often.
• Include proper header information on all e-mail messages.
• Do not send messages that result in too many concurrent simultaneous connections to our mail servers.
• Do not attempt to discover e-mail accounts by sending messages to the AT&T mail server. Servers making this attempt will be blocked by AT&T and further legal action will be taken.