SAP Mail Stuck and give error Msg 816

Dear Experts ,

We are using SAP ECC6.0 on HP Unix 11.31 with oracle 11g ,from SAP so many Internal and External mails goes to users.

But some mails stuck in SAP give Error Msg 816 .

I have check our mail Server , Network all is working fine .But these SAP mails give error occasionally .

What and why this error come , Please find the screen shot for detail.

It will be great full for me.

Regards