The CAN-SPAM Act of 2003 (15 U.S.C. 7701, et seq., Public Law No. 108-187, was S.877 of the 108th United States Congress), signed into law by President George W. Bush on December 16, 2003, establishes the United States’ first national standards for the sending of commercial e-mail and requires the Federal Trade Commission (FTC) to enforce its provisions.

from Wikipedia

Depending on your country and it laws you have to apply certain rules to prevent getting sued for your sent newsletters. If you’re located in the U.S. the CAN-SPAM act applies to all your outgoing mail. In other countries there are similar laws: