While unsure, I think this description from the FastMail Help is accurate:
Quote:
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Auto-purge - Automatically delete messages after they have been in the folder for a certain amount of time, from 1 day to 1 year. Pinned messages are never deleted.
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Note that the test is not on the age of the message, only the length of time the message has been in the folder. Thus, a message from May would remain in the folder if it was only moved to the folder recently.