I noticed that there are two fields for the date/time of the emails: one is called date and the other is called received (in the italian translation they are Data and Ricevuto as you can see in the attachment).
In the POP3 configuration (see Screenshot POP3 in the attachment) the Data field represents the data/time of the message as downloaded by Thunderbird) whilst the Ricevuto field represents the real date/time the mail server has received the message by sender. In the atthacment you can see that the subject includes the date/time the message was sent and you can compare it with the Ricevuto field.
If I configure the same account in IMAP the two fields, (see Screenshot IMAP in the attachment), represent the same datat/time and it is the time at which the message was downloaded by Thunderbird and the actual sending date cannot be seen.
I don't know it it is a problem of Thunderbird or if it is a problem of the mail provider but the webmail, of which the IMAP shoud be a copy, shows the date/time at which the server received the message from the sender which, of course, is different from what is seen in the IMAP screen (see Screensho WEBMAIL in the attachment).
Could you investigate whether in the IMAP configuration there is a mistake in the usage of the two fiels Date and Received or, on the contrary, confirm that is the provider of the mail that is making the mistake?
Many thanks
Aldo
