Exchange 2013 to 2010, email stuck in queue when send to large distribution group

Hi,

We have a exchange 2010 sp3 cu10 and exchange 2013 cu9 and migrate some user from 2010 to 2013

When exchange 2010 user send to  a distribution group (which all members still in 2010), it works

But when exchange 2013 user send to distribution group:

- if the group only have few memebers, it can send to target mailbox

- if the group have many memebers (more than 1000), email will stuck in exchange 2013 and cannot route to exchange 2010. The error is "421 4.4.1 Connection time out"

Tried to change the receive connector timeout setting, and increase recipient limit by set-transportservices command but still have problem

Any idea?

Thanks

Andy Chong