Amazon says this:
Quote:
Keeping Subscription Content
Older issues of newspapers and magazines appear inside a grouping called "Periodicals: Back Issues." Selecting the grouping takes you to a screen that displays the back issues you have on your Kindle.
Kindle automatically deletes issues that are more than seven issues old to free up space for new content. An exclamation point next to an issue indicates that it will be deleted soon. If you'd like to keep a copy of an issue, follow these steps:
To keep a back issue:
Open the specific issue from the Kindle Home screen.
Press the Menu button and select "Keep This Issue."
You will then see the word "keep" to the left of the issue title in Home.
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But I have no idea if that will apply to calibre subscriptions. I have noticed that unless I delete them, calibre subscriptions start going into the "Periodicals: Back Issues" grouping as well. I'll avoid deleting one of my feeds and send it a few times to see what happens. Might take awhile however...