Someone on the mailing list has a spam filter that is auto-replying when it blocks. Trend Micro Spam filter appears to be the name.

If you use Trend Micro Anti-Spam (apparently the German version), please turn off the settings causing it to auto-reply whenever your local client rejects a message on the mailing list.  It is causing people that post to the mailing list to believe they are being rejected when they are not.

Here is a sample output from your client (that really hates Gmail senders)...

> The following mail was blocked since it contains sensitive content.
> 
> Source mailbox: aaabbbccc-Re5JQEeQqe8AvxtiuMwx3w@xxxxxxxxxxxxxxxx
> Destination mailbox(es): fw-general-wwOhfgvld6hpLGFMi4vTTA@xxxxxxxxxxxxxxxx
> Rule/Policy: Trend Micro Anti-spam
> Action: Quarantine to
> C:\Programme\Trend\SMCF\Quarantine\2006-06-12\13\41\Message448d52eb9da.origina
> l_eml_
> 
> Absender, Trend Micro Anti-Spam hat eine vertrauliche E-Mail entdeckt.