How to use complexe "from" on emails?

I've seen this on an email received from Belgian tax administration.

The from field is composed of two email addresses :

The one of the employee who's in charge of this case and who have send the email
General one for the department which this employee is part of

And in between these two "senders", "de la part de" (French) which would probably be written "on the part of" in English but I haven't found any other exemple (whatever the language).

So I'm curious to know how to organize email header to achieve this result, but once again I haven't found anything about. Or maybe I haven't google it properly...

Solution

Listing several mailboxes in the From header field is valid according to RFC 5322:

The from field consists of the field name "From" and a comma-separated list of one or more mailbox specifications. If the from field contains more than one mailbox specification in the mailbox-list, then the sender field, containing the field name "Sender" and a single mailbox specification, MUST appear in the message.

If you want a more detailed analysis, post the headers of the raw message rather than a screenshot.