Now I've found another solution:
An LDAP object in Exchange may have the following multivalued attribute: Auth-Orig
If you put a senders dn here, you can allow this sender to post to that distribution list.

EG:

dn: cn=$dlName,cn=$server,o=$domain
objectClass: groupOfNames
objectClass: person
objectClass: Top
rdn: $dlName
cn: $dlName
distinguishedName: cn=$dlName,cn=$anything,ou=$server,o=$domain
Hide-From-Address-Book: FALSE
rfc822Mailbox: $email@$server.$domain.com
mail: $email2@$server.$domain.com
name:  Strat Mailinglist (simpl-name)
Hide-DL-Membership: FALSE
uid: $dlName
OOF-Reply-To-Originator: FALSE
Report-To-Originator: FALSE
Report-To-Owner: FALSE
Auth-Orig: cn=$anyCn1,cn=Recipients,ou=$server,o=$domain'
Auth-Orig: cn=$anyCn2,cn=Recipients,ou=$server,o=$domain'
Auth-Orig: cn=$anyCn3,cn=Recipients,ou=$server,o=$domain'
[download]

$server, $domain and so on were just replaced by me for not to have to print real data ;-)

Best regards,
perl -e "print a|r,p|d=>b|p=>chr 3**2 .7=>t and t"