This is the second time that Kmail deleted all my mails. I always use thunderbird but want to give kmail a try since I like KDE.

One year ago I tried kmail but accidentally deleted all my mails when I simply wanted to delete the account. OK, maybe that's my misunderstanding.

Now this time, I simply used the function "mark all mails as read", today I found all my 400+ mails have been deleted. Even when I look into the trash box on the mail web service, it's empty. Now I can only say that, I WILL NEVER TOUCH KMAIL IN MY WHOLE LIFE AND TELL EVERYONE I KNOW TO AVOID THE JUNK PROGRAM. The first thing I do after installing KDE will be uninstalling Kmail, as it is the most dangerous program I've ever used in my whole life.

If only anyone can tell me if it's possible to recover my mails.

First of all, I'm sorry for your issue. I'll need more information on it to understand if mails are recoverable.

What version of KDE PIM are you using?
What type of account are you using (POP, IMAP....)?
Did you mark as read messages in a local folder, or in an account?

It's POP3.

I unplugged my network and opened both kmail and thunderbird on the laptop where I used "mark as all read" function.

The mails disappeared on the mail server are still here on my computer. How do I recover these mails to the server? I believe if I run either kmail or thunderbird with internet the mails would be deleted immediately.

What version of KDE PIM are you using? Are you sure that KMail was configured to keep mails on the server? I don't recall (having used POP3 only a long time ago) if it's the default.

Either case, both Thunderbird and KMail should have your mails (one in mbox format, one in Maildir format). I'd need to know your PIM version to be sure, but for KMail they should be be in ~/.local/share/local-mail or ~/.local/share/.local-mail.directory/ (or both).