iCloud
iCloud User Guide
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Welcome
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Messages
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More resources

If your iCloud mailbox is full
By default, Mail uses IMAP for your iCloud email account. If your iCloud storage fills up, you won’t be able to receive new email until you make more iCloud storage space available. You can do any of the following:
Delete iCloud Mail messages. See the Mail section of the Apple Support article Manage your iCloud storage.
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