Exchange 2013 mail stuck in Queue

Hi,

I installed Exchange server 2013 CU9 on windows server 2012 R2. The servers were specified with one CAS and one mailbox server and one windows server 2012 R2 as a SMTP server with Exchange POP3 connector. Local email communication is working fine and don't have any issue. The configuration made that email receives through POP3 connector on SMTP server and working fine and can receive external emails. The send connector is configured as smart host and assigned SMTP server IP address. The problem I am facing that when I send email it gets stuck on Exchange 2013 mailbox server queue and generate error ";{LED=441 4.4.1 Error encountered while communicating with primary target IP addrress; "Failed to connect. Winsock error code: 10060, Win32 error code 10060. Attempted failover to alternate host, but that did not succeed." Can some one guide what is the actual root cause that emails are not delivering to the SMTP server and getting stuck in Queue.