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IMM Issue 34 - International Mail Manual >
2 Conditions for Mailing > 240 First-Class Mail International
The First-Class Mail International classification encompasses all the classes of international mail that were formerly categorized as airmail letter-post and economy letter-post, post and postal cards, printed matter, and small packets that were formerly categorized as LC (letters and cards) and AO (other articles).
Any article that is otherwise acceptable and not prohibited by the country of destination, subject to applicable weight and size limits, may also be mailed at the First-Class Mail International rate.