Sorry, I haven't made myself clear enough. This is not happening in the front end, in fact it's not related to any form fields, at all. I just log into the backend, create a new form, go into the form options and enter a mail address into the "Email to send submissions to" field:
This will prompt the error:
"This is not a list of valid email addresses."
If I remove the ampersand, altering the address to
everything works fine.
This might be related to some standardized mail address evaluation routine? The ampersand seems not to be supported on every server and php version, but it is officially allowed in mail addresses and, so far, has worked perfectly fine for me.