Short answer: your emails live on your mail provider's server, and clearing the cache only removes the copies BlueMail downloaded to the device. They re-download the next time the app syncs. But "clear cache" and "clear data" are very different actions — here is exactly what each one touches.
Clear Cache inside BlueMail — safe
Settings | Global Settings | Advanced Features | Clear Cache removes the locally stored copies of your messages; BlueMail then re-downloads them according to your Offline Storage settings. Nothing is deleted from the server, and your accounts and settings stay untouched. This is the standard fix for things like a wrong unread count — see How to Clear Cache.
Android's "Clear cache" — safe
Android Settings → Apps → BlueMail → Storage → Clear cache removes temporary files only. Accounts, settings, and mail are unaffected.
Android's "Clear data" / "Clear storage" — a full reset
This one is different: it deletes all of BlueMail's local data — your accounts, your settings, and every locally stored message — and returns the app to a fresh-install state. Your mail is still on the server, so after re-adding your accounts it syncs back, but your in-app preferences are gone. (On the desktop app, Settings | Global Settings | Advanced Features | Clear App Data does the same thing.)
The one real risk: POP3
With a POP3 account, messages can exist only inside the app — for example when they've been downloaded and later removed from the server, by your settings or by another device. Those local-only messages have no server copy to re-download: a data clear (or uninstall) deletes them permanently. If you use POP3, check the account's Delete Messages on Server setting before any kind of reset.
Attachments
Attachments you saved to your device (e.g. to Downloads) are files of their own and survive any of the above. Attachments you only ever opened inside BlueMail are part of the message and follow the same rules as the message itself.
