Hi there. I have a problem sending emails to Gmail. My server is an Exchange 2013 and we have an AD forest. Also, we have a proxy server with 2 ISPs. I can send messages through one of the ISP. However, when I switch to another ISP service, I get these messages:
returned '441 4.4.1 Error encountered while communicating with primary target IP address: "Failed to connect. Winsock error code: 10051, Win32 error code: 10051." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.
My server Exchange 2013:
1-. It can resolve any domain (gmail.com for example) through any of my ISP providers.
2-. My DNS server is my PDC server.
3-. Port 25 on the proxy is opened and working. The tests I did shows that SMTP services over any of my ISPs are working well.
4-. Both servers (PDC and Exchange) have static IPs.
5-. MX records are registered in the DNS zone.
I have no idea why we can't send emails using one ISP and with the other services, emails flow well. Here is a message from my server indicating a delay:
I noticed that in the message above, the address is IPv6. But I don't have any IPv6 Network running. I don't know if this could be the problem. All this is happening after the ISP with the issue was listed as a Spammer (not my IP address but the whole segment) and then delisted.
If someone can help here, I will be appreciated it.
Regards