How far back does the Spam Folder hold emails? Can I retrieve a spam email from a month ago?

Spam is stored for 7 days or 2,000 messages, whichever comes first
Trash is stored for 14 days or 5,000 messages, whichever comes first

Messages that drop out of  Spam cannot be recovered.

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@Latoque wrote: ... Messages that drop out of  Spam cannot be recovered.

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My experience is that when the system deletes messages from the webmail Spam folder after 7 days, they are moved to "Recover Deleted Items".