Emails are not reliably sent, a further notice (Technics)
It reads like this:
From: Text@dd39012.kasserver.com, Forum@dd39012.kasserver.com,
nicht =?UTF-8?Q?=C3=B6ffentlich=2C=20nur=20f=C3=BCr=20Mitg?= =?UTF-8?Q?lieder?= <Forum <email@example.com>>
Beside the problems, described in the other reply, I see another problem, when reading the RFC 2822 that describes the format of a valid e-mail. But, to say it in the beginning, that's a point we have not in our hand.
Section 3.6.2 of RFC 2822 (you have to search it with the browser search tool (normally to open with [CTRL]+[F]) describes the headers, that can be set to specify the sender. There are following possible headers and their quoted descriptions (wit additonal notices from me).
from = "From:" mailbox-list CRLF
This can be one address or mailbox or a list of addresses and/or mailboxes.
The "From:" field specifies the author(s) of the message, that is, the mailbox(es) of the person(s) or system(s) responsible for the writing of the message.
sender = "Sender:" mailbox CRLF
This can be one address/mailbox but not a list.
The "Sender:" field specifies the mailbox of the agent responsible for the actual transmission of the message. For example, if a secretary were to send a message for another person, the mailbox of the secretary would appear in the "Sender:" field and the mailbox of the actual author would appear in the "From:" field. If the originator of the message can be indicated by a single mailbox and the author and transmitter are identical, the "Sender:" field SHOULD NOT be used. Otherwise, both fields SHOULD appear.
reply-to = "Reply-To:" address-list CRLF
This can be one address or a list of addresses but not mailboxes.
When the "Reply-To:" field is present, it indicates the mailbox(es) to which the author of the message suggests that replies be sent. In the absence of the "Reply-To:" field, replies SHOULD by default be sent to the mailbox(es) specified in the "From:" field unless otherwise specified by the person composing the reply.
The descriptions are followed by this sentence:
In all cases, the "From:" field SHOULD NOT contain any mailbox that does not belong to the author(s) of the message.
That means, when I do not read and understand it completely wrong, that the additional addresses that got added by your provider, would have to be appear in an added Sender-header but NOT in the From-header because they in fact "does not belong to the author(s) of the message" and it has to be only one address. But that's another issue for your provider.
Trenne niemals Müll, denn er hat nur eine Silbe!