Postmasters may install bulletin boards in public areas of Post Offices or set apart a convenient display area. Displays must adhere to the following guidelines:

• Displays are for government notices, notices of public assemblies, judicial sales, official election notices issued by state or local government, and similar announcements.
• These display privileges must be afforded without discrimination.
• Displays may not include private business advertisements, such as placards, cards or notices.
• Displays may not include photographs or likenesses of any member of state legislature or Congress; or political pictures, cartoons or other material designed to influence an election.
• Notices by members of Congress are limited to public announcements of pending competitive civil service examinations and pertinent information about application for appointment to the U.S. service academies.
• If space is available, one recruiting notice (no larger than 8-1/2 in. by 11 in.) of each branch of the armed services may be displayed.

For a complete list of all guidelines, refer to section 125.361a of the Postal Operations Manual.

via USPS News Link Story – Postal bulletin board guidelines.