I've just found something interesting while checking the email header.

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 <info@domain.de>>

However, the email in the settings reads:

Forum <info@domain.de>

Why this? The settings should only contain the e-mail-address in itself, not a text like Forum <mail@example.com>. The forum script does the stitching of the forum name and e-mail-address by itself.

The title of the Forum is set to:

Forum, nicht öffentlich, nur für Mitglieder

After changing this to just "Forum" it seems to work.

What am I missing here?

The questionable setting above led to an invalid From-header. The header field From contains a comma separated list of e-mail-adresses (From: info@example.com) or mailboxes (Forum <info@example.com>). Your forum name stands alone for itself and breaks the necessary format. It must be: Forum <info@example.com>, but is (according to your posting): Forum name <Forum <info@example.com>>.

That's documented in your own example. We will inspect it from outer to inner side. The next code block contains the whole content of the From-header. The first two addresses got added after delivering the e-mail with the function mail to the mail relay of your provider.

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 <info@domain.de>>

If we remove the two addresses that got added by the mail relay (these are technically valid entries) we will get the following.

nicht =?UTF-8?Q?=C3=B6ffentlich=2C=20nur=20f=C3=BCr=20Mitg?= =?UTF-8?Q?lieder?= <Forum <info@domain.de>>

The first part is the Name of your forum, encoded to be transported in UTF-8 followed by the generated output with your broken setting.

Only the forum name:

nicht =?UTF-8?Q?=C3=B6ffentlich=2C=20nur=20f=C3=BCr=20Mitg?= =?UTF-8?Q?lieder?=

Now to the broken part of the sender mailbox.

The name (set by the forum script and shown above) is followed by the wrong setting for the e-mail-address of the forum. The outer brackets got added in the function my_mail, the inner brackets was added to the wrong formatted setting of the address by you.

<Forum <info@domain.de>>

With a clean setting for the forum e-mail-address (info@domain.de, not Forum <info@domain.de>) the mailbox entry would look like this:

nicht =?UTF-8?Q?=C3=B6ffentlich=2C=20nur=20f=C3=BCr=20Mitg?= =?UTF-8?Q?lieder?= <info@domain.de>

Going back to

After changing this to just "Forum" it seems to work.

This is a second possible problem. The (old, long) name contains commata. Commata separates the entries in the From-header, so the From-header contains the following sender entries (not encoded):

- Text@dd39012.kasserver.com
- Forum@dd39012.kasserver.com
- Forum
- nicht öffentlich
- nur für Mitglieder <Forum <info@domain.de>>

The senders Forum and nicht öffentlich are invalid, the last entry nur für Mitglieder <Forum <info@domain.de>> anyway. I would expect the forum name to be safe because of its encoding. But it not seems to be so.

Please try sending e-mails with a forum name with more than one word but without commata and a corrected setting of the forums own e-mail-address (only the address in itself).

Tchö, Auge