I'm getting a bad feeling about Gmail. I've used it for years and recently I deleted all of my private conversations from all possible devices and folders. However, when I go to the search box of Gmail and look up specific keywords in previous mails, they pop up. I'm not actually able to open them, but I can read the first lines everywhere. Am I overlooking something or is it really NOT possible to permanently delete mails? Is Google forced by law to use soft delete? What do others do?
Gmail never actually deletes emails
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Almost all websites cache of data on your local device. With Gmail they are updating all the changes since the last update. Email could have been sent and received. Messages could have been labeled or archived. Some of these changes take place behind the scenes.
Remember many people use Gmail on multiple platforms. It is not unusual to have people logged into the service on the work computer, home computer, and phone at the same time.
All this is made even more complicated if the user has decided to use the offline option, where a copy of the data is on the machine, so that the user can read and manage email when they don't have internet access.
Finally, I have found source of the problem and want to share it for others, having the same problem.
It turned out, that I accidentally unchecked Treat as an alias checkbox for my main account, on Settings → Accounts and Import → [account name] → edit info. After checking it back (this time for all my accounts) entire Reply functionality got back to normal.
Turns out, that this is not a bug, but some kind of feature, about which Google is aware and have it described in details. Most important parts:
If you use Gmail's Send mail as feature to send messages on behalf of another email account, you may encounter the following issues: (...) If you receive a message from a Send mail as address and you click Reply, the To: field is incorrectly populated with your primary address. (...) These issues occur because Gmail treats your Send mail as address as an alias of your primary address. You can change this behavior in your mail settings by deselecting Treat as an alias.
And now, comes the best part. Either I'm missing something, or in my case the solution was to do something exactly opposite to what Google suggests! From my understanding of above text (correct me, if I'm wrong), I should deselect Treat as an alias to fix the problem, that To: field is being incorrectly populated with my primary address (that is exactly, what was happening to me).
Well, the problem is, that I was experiencing above mentioned problems when I had Treat as an alias checkbox unselected. And selecting it back solved the problem. Only after then my To: field started to become correctly populated—exactly how it is described in Kevan Sheridan's answer.
Best Answer
Deleting from any folder except Spam or Trash moves the message to Trash, which can still be searched (similar to the Trash or Recycle Bin for files on a desktop OS). Deleting from one of those two folders deletes it permanently, and such messages will no longer show in search results. (I just tested this to double-check).