Hello I'm opening a new thread for this as Ive got stuck again.

I have some users that try to send mails to about 5 domains that wont succeed. 99% of al mail is going out like it should but to these domains all messages get stuck in the queue. The NDR they get is :

Remote server at domain.com (x.x.x.x) returned '400 4.4.7 Message delayed'

Remote server at domain.com (x.x.x.x) returned '441 4.4.1 Error encountered while communicating with primary target IP address: "Failed to connect. Winsock error code: 10060, Win32 error code: 10060."

Attempted failover to alternet host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts. the last endpoint attempted was x.x.x.x:25

Most posts regarding this error suggest a DNS issues and it does look like this. but when I do an NS lookup on the server the correct MX records are being returned. I have also tried with the use External DNS option but no luck. other things Ive tried is to use a Send connector with TLS, use different ports but they wont go out. I've contacted the admin of the receiving domain and they say they have not blacklisted our IP's. So I'm really stuck here any input would be very welcome. FYI: I've check the blacklists and we're not on them, I have reverse DNS setup and SPF records and with the mxtoolbox all checks out fine.  

• Edited by dimiwave Tuesday, May 05, 2015 9:59 AM wrong error code

Hi thanks for the reply,

I use Exchange 2013 SP1 CU8,

There is no SMTP filtering enabled I use Cisco ASA5525X with 9.1 software did some debugging and nothing is being stopped with at the firewall.

We dont have antispam filtering enabled,

I cant connect to any of the remote hosts by telnet have tried, but can sent emails to them via gmail, the receiving client is using a Symantec Encryption Management Mail proxy as I understand but other than that only our emails seem to be blocked.

I also today tryed a different Edge Transport Server on an other external subnet but same error there.

Since the telnet is not successful then there is some real problem 

I think then there is some transport agent or firewall causing this issue

Else better reach out an admin on that company and ask him to add your connecting IP as allow list in his Symantec Encryption Management Mail proxy 

Also you can try by creating a dedicated send connector for one of the affected domain and Smart host it to thier IP and see the results