Deceptive Mailings Prevention Act of 1990

Deceptive Mailings Prevention Act of 1990 - Amends Federal law to declare nonmailable any mail matter which constitutes a solicitation by a non-governmental entity: (1) for the purchase of or payment for products or services and which contains a seal, insignia, trade or brand name, or any other term or symbol which reasonably could be interpreted or construed as implying Federal Government connection, approval, or endorsement; and (2) for information or the contribution of funds or membership fees and which contains a seal, insignia, trade or brand name, or any other term or symbol which could reasonably be interpreted or construed as implying Federal Government connection, approval, or endorsement. Allows the mailing of such matter if it contains a conspicuous disclaimer that it is not a Government document.

Imposes penalties for violations of this Act.

Requires the United States Postal Service to conduct comprehensive reviews to determine: (1) whether or not certain provisions of the Domestic Mail Manual relating to the authority of postmasters with respect to the mailability of matter are being properly observed; and (2) the feasibility of establishing a procedure to obtain a final agency decision through expedited proceedings as to the mailability of mail matter which is denied entry into the mails on the basis of incorrect mail preparation, postage due, or addressing. Directs the Postal Service to: (1) take appropriate measures to ensure that any misapplication or misunderstanding of such mailability provisions is corrected; (2) report to the Congress on its findings and actions regarding such reviews; and (3) coordinate the administration of this Act with the Secretary of Health and Human Services pursuant to the Medicare Catastrophic Coverage Act of 1988.

Authorizes the Postal Service and the State Department to test the feasibility of providing postal services through State Department personnel at overseas diplomatic missions and to establish branch post offices at overseas diplomatic missions at locations where postal services are not provided at armed forces installations. Permits the establishment of such branch post offices if it is determined feasible and in the public interest.