active sync works fine; the mobile devices receive mails and it is o.K. to send mail to our own domain. It is also o.K. to reply to mails from other domains.

It is not possible to send a NEW mail to an account outside our own domain. All issues occur both to iPhones and Android devices; that seems to be a problem of the server and of the mobile devices.

DNS etc. on the server is o.K.; otherwise we could not send from the desktop-clients.

We have no idea why there is a difference between sending from the mobile device and a "normal" client.

Any Ideas ?

Thanks ...