thanks! I successfully uploaded the new version. Including the UTF-8 encoding in $header is a good idea. Before this modification we had observed that some messages were fine, others had errors. This may have been due to the device (Mac, PC, smartphone…) that the individual visitor of the website had used. Now, it seems pretty good… I tested it by sending messages from different devices to different email addresses and reading these emails with different mail clients. The problem seems to be resolved.
Two final points…
First, I was confused by the fact that the messages were displayed incorrectly (even now) when I looked into my gmx-account (IMAP) via different browsers. The mail clients, however, that download the messages from the gmx server displayed the messages correctly. No, I do not want to talk with gmx about the encoding of their webmail interface ;-). But it shows that testing such problems can be complicated.
Second, I noticed that the [contact] shortcut in the Trattoria theme sends the email to the address given in WordPress-Settings-General-Email address which is marked as “Only used for administrative purposes”. The email address given in Design-Theme options-Contact Page is simply ignored. Is this a feature or a bug?