Who is your mail provider for the affected account? On the Folder Pane, right-click the account name and select Subscribe. The Subscribe dialog will open showing the list of folders and their subscription statuses. Select ALL listed folders by clicking on the first folder, scroll down to the last, hold Shift while clicking the last folder then release Shift. Click on Unsubscribe then click OK to close the dialog. Reopen the Subscribe dialog, select ALL listed folders then click Subcribe and click OK to close the dialog.

Now open Account Settings > Copies & Folders > Place a copy in > Other then click inside the drop-down field, select your account and navigate to that account's Sent folder. How that folder is called depends on your mail provider. Some may simply name it Sent, others may call it Sent Mail and so on. Google (Gmail), for example, calls it Sent Mail. Untick the option Place replies in the folder of the message being replied to then click OK to save the changes and close the dialog. The fact that this is happening on multiple devices for the same account makes me think it must be a problem with the account's default configuration on Thunderbird.

Sorry, I made a mistake.

The main account is POP. It stores an offline copy of the emails, but it also leaves them on the server unless they're deleted.

The secondary ones, instead, are IMAP.

What I never managed to do is send emails from the IMAP ones and make them appear in the other ones too. It's like they're all separated somehow..

--

Anyway, I tried your suggestions, but nothing changed, sadly. And "Place replies" was already unticked.

Umm.. I checked the webmail online and I noticed that the default localized "Posta Inviata" (Send) isn't there!

There's just a custom "Send" folder, containing only the test email created by one of the IMAP accounts.

If I try sending one directly from the webmail, it doesn't appear anywhere. Maybe the mail provider messed up! O_o I'll contact them, thanks.