exchanger 2013 about time out problem

My hk office using a outlook , and it have a Exchange server in china.

We only use the Internet line for the connection.

Recently, if we send out a msg with a attachment, the msg still hold in users outbox with over 5mins...(it show sending ..)

then after a while, the msg can send out, but in send box,it will show me double, and receipt will get 2 times too

I think it may because connection slow or drop, but can Server just send out the msg 1 time? not keep retry for 2-3 time to send out?

thanks

做左PTR 未