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 21 hours 32 minutes ago wrong error code

Try dropping an email through telnet to one of the affected domain and see if its successful or if it gets failed in any transit

What version of Exchange server you are using ?

Do you have any SMTP filtering enabled on your Firewall ?

If so please check if this SMTP filtering is causing this issue 

Do you have any Antispam filtering on your environment for outbound?

If so please check in your Antispam filtering logs if its blocked for any reason

enable protocol logs on all the connectors and try dropping an emails to those affected domains and see if you get any information on the logs