iCloud mail disappears after 30 days

If storage is full mail will not be stored on server / device , How to check the storage on your iPhone and iPad - Apple Support

Manage your iCloud storage - Apple Support

Else the mail application is not working , remove the mail account and once again add If you can’t send email on your iPhone or iPad - Apple Support

Remove your email account and set it up again

1. On your computer, sign in to your email provider’s website. Make sure that all of your email is there or make sure that your email is saved somewhere other than on your iOS or iPadOS device.
2. On your device, go to Settings > Mail, then tap Accounts.
3. Tap the email account that you want to remove.
4. Tap Delete Account.
5. Add your account again.

If the steps in this article don't help, contact your email service provider for more information.

Note : you can look your mail settings Mail – Official Apple Support

e_kaz Said:

"iCloud mail disappears after 30 days: I just noticed today that all of the emails in my iCloud account have disappeared beyond 30 days.[...]"

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Troubleshooting Lost eMails:

I. Check iCloud Mail, Directly:

Log in here and see if any iCloud eMails are seen: https://www.icloud.com/mail

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II. Retrieving eMails:

A. Open Other Folders: Look in the Trash ro Archived folders. See if they reside in there.

B. Check Other Device's Mail App:

• If Mail was enabled on your other devices (i.e. Mac) along with your iPhone computer, then check and see if the Messages app is enabled on it to see if it exists in it. For a Mac, you'd start up your Mac, not connecting to the Internet, and open the Mail app. Then proceed from there with retrieving the eMail messages.
• IMPORTANT: Be certain you do not connect to the Internet while booting up, as syncing would delete it. So, power off your router to avoid the connection. Go here to Learn how to Export it: Import or Export Mailboxes in Mail on Mac - Apple Support

e_kaz Said:

"iCloud mail disappears after 30 days: Unfortunately this did not work.”

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They are gone :(

Without that having worked, then, they are gone. So, take this as a learning experience, knowing to alway: keep a backup of your Mailboxes. Go here to Learn how to Export it: Import or Export Mailboxes in Mail on Mac - Apple Support

See the article Delete email in Mail on iCloud.com - Apple Support

Deleted messages are moved to the Bin mailbox. When you delete a message, it is saved in your Bin for 30 days, after which it is permanently erased. When you delete an email from iCloud.com, it is deleted on all your devices that have Mail turned on in iCloud settings.

You can also change where your deleted messages are stored.

Important: If you view iCloud Mail in another app (for example, the Mail app on your iPhone, iPad or iPod touch), your mail might be deleted sooner than 30 days depending on the app’s settings.