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In the early days of postal communications it was often necessary for international mail to pass through a number of hands before reaching its eventual destination. At each stage the agent would add their own mark. For instance, a letter might pass first through the sender's domestic post office's hands, then to a forwarder for a sea journey and then to the post office of the destination country.
The study of the marks of forwarding agents on mail is a popular branch of postal history.
- Rowe, Kenneth. The postal history and markings of the forwarding agents. (1st edition 1966, supplement 1974, 2nd ed. 1984, 3rd ed. 1996. ISBN 0-917528-12-3)