The franking privilege, which allows Members of Congress to transmit mail matter under their signature without postage, has its roots in 17th century Great Britain.

The British House of Commons instituted it in 1660 and free mail was available to many officials under the colonial postal system.

In 1775, the First Continental Congress passed legislation giving Members mailing privileges so they could communicate with their constituents, as well as giving free mailing privileges to soldiers.

Congress continues to use the franking privilege to satisfy an articulated public interest in facilitating official
communications from elected officials to the citizens whom they represent.

The communications may include letters in response to constituent requests for information, newsletters regarding legislation and Member votes, press releases about official Member activities, copies of the Congressional Record and government reports, and notices about upcoming town meetings organized by Members.

Member Mail Allowances
Congress pays the U.S. Postal Service for franked mail through annual
appropriations for the legislative branch. Each chamber uses a formula to allocate funds to Members from these appropriations.

In the Senate, the allocation process is administered by the Committee on Rules and Administration; in the House, by the Committee on House Administration.

In the Senate, each Senator’s franked mail postage allowance is determined by a formula that gives a maximum allowance equal to the cost of one first-class mailing to every address in the Senator’s state.

If the total Senate appropriation for official mail is less than the amount required for the maximum allowance, each Senator’s allowance is proportionally reduced.

A Senate office that exceeds its allowance may supplement the allowance with official office account funds.

Senators are, however, limited to $50,000 for mass mailings (defined as 500 or more identical pieces of unsolicited mail) in any
fiscal year.

In the House, the franked mail postage allowance is based on the number of
addresses in each Member’s district.

Each Representative’s mail allowance is combined
with allowances for office staff and official office expenses to form a Member’s
Representational Allowance (MRA). Members may spend any portion of their MRA on
franked mail, subject to law and House regulations.

Within the limits of their MRA, House Members are not restricted as to the total amount they may spend on mass mailings.

Above from the Congressional Research Service, Library of Congress