Today i get a answer from Polo about my stucked trans:

Apologies for the late response.

The transaction was not rejected by the network its just that it hasn't confirmed as of yet.

https://minergate.com/blockchain/xdn/transaction/ef3a763cb884a19af6231c0595af4b25ee2edd3b16454e1ef14f48a6d3e74c5f

As you can see there is an ample amount transaction fee on this and I'm afraid this is now at the mercy of the network. Rest assured we'll keep an eye on this and we'll broadcast it again should it be necessary.

Best regards,

Jerichk
Poloniex Support