Parkside is a great option on that front as Rod pointed out above, but you can also reach out to our sales team for a recommendation to one of our many partner organizations. In the meantime, here's some helpful resources on LDAP/Active Directory integration and importing users:

https://portal.smartertools.com/kb/a2693/enable-ldap-or-active-directory-for-a-domain.aspx?KBSearchID=785395 https://portal.smartertools.com/kb/a2713/import-new-users-using-ldap.aspx?KBSearchID=785395 
Lastly, here are some resources detailing the creation of mailing lists, and the upload/import of mailing list subscribers:

https://portal.smartertools.com/kb/a2888/create-a-mailing-list.aspx?KBSearchID=785396 https://portal.smartertools.com/kb/a2796/upload-multiple-subscribers-to-mailing-lists.aspx?KBSearchID=785396 
Hopefully those get you going in the right direction :-) If you have any specific questions please don't hesitate to reach out to support or post in our community for other user's recommendations. Thanks for choosing SmarterMail!