That error:
451 4.4.397 Error communicating with target host. -> 421 4.2.1 Unable to connect -> SocketTimedout: Socket error code 10060
means your Exchange server (or another sending SMTP server) tried to deliver an email but couldn’t establish a TCP connection to the recipient’s mail server, even after DNS resolution. Let’s break it down:
🔍 What It Means
- 451 4.4.397 — A transient SMTP failure. Your server will likely retry.
- 421 4.2.1 — Recipient’s server (or an edge SMTP device) is unavailable.
- Socket error code 10060 — A timeout occurred while trying to connect via TCP. It means your server sent SYN packets, but got no response in time.
🛠️ Likely Causes
- Recipient server is down or refusing connections
- Temporarily offline
- Overloaded
- Port 25 is closed or firewalled
- Firewall or network issues on your side
- Your server might be blocked from accessing outbound port 25
- Network routing issues (especially if this is specific to one recipient)
- Anti-spam filtering
- Some providers (like Outlook.com, Gmail, etc.) may drop or silently reject traffic from IPs on certain reputation blacklists or missing reverse DNS records.
- Misconfigured DNS
- DNS resolution may be returning the wrong IP, or no MX/A record at all.
✅ What to Check
🔁 1. Retry from the queue
Exchange may retry automatically, but you can manually re-queue:
Get-Queue | Get-Message | Retry-Queue -Resubmit $true
🌐 2. Test connectivity to the recipient’s MX
nslookup -q=mx domain.com
Then:
telnet mx1.domain.com 25
or
Test-NetConnection -Port 25 -ComputerName mx1.domain.com
🔒 3. Check if you’re being blocked
Use online tools:
📜 4. Review Exchange logs
Look in SMTP Send Protocol Logs for the exact timestamps and handshake attempts.
🔃 5. Try from another server
Ping or telnet from another machine or VPN to isolate whether it’s your server or a general issue.
🚑 Temporary Workaround
If the email is critical and you control both sides, consider setting up a smarthost to relay temporarily. Or use another email service (like Outlook or Gmail) to contact the recipient.
Let me know the recipient domain or if you want help running diagnostics from your side (e.g. test commands or logs to check).
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