Instead of copying the backup messages directly into my email account folders I used the import procedure. After that, I selected all messages and moved them from the imported folder to my account. And all the displayed information was correct and complete.

I tried before to use the import procedure but when the wizard asked for the location of the messages I wrongly chose the Sent items folder or the Inbox folder. This is not accepted by the wizard. Instead I should have chosen the account folder which contains the Sent items folder and the Inbox folder.