Member Avatar for lsutiger

Hi all! I have been scouring the net loooking for some help with a problem our company is having.
We have our own smtp/pop3 server setup in house. We are using Dyndns' mailhop service in order to control spam, and it works! When someone sends our domain an email, it is forwarded to their servers, scanned and then forwarded to our server.

But lately we have been receiving our emails anywhere from one to several hours late. I looked at the headers for a bunch of messages and they are getting from point a to point b in a timely fashion, but from point b to c is the problem.

When I contacted Dyndns' tech support, they stated that their logs show that there were connection timeouts when trying to contact our server.

My question is how to troubleshoot this problem and find the point of failure.

Thanx in advance!

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  • Latest Post Latest Post by DimaYasny

Dani AI

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A focused checklist to close the gap between DynDNS Mailhop and your IIS6 SMTP server, and how to lock SMTP to only accept Mailhop hosts.

As found, NAT/port translation behavior on cheap routers can cause intermittent timeouts. Combine server-side logging and packet captures with host‑level access controls so you can both diagnose and prevent unwanted connections.

  • Restrict IIS SMTP (Windows Server 2003 / IIS 6.0)

    1. Open Internet Information Services (IIS) Manager.
    2. Right‑click the Default SMTP Virtual Server (or your SMTP instance) -> Properties -> Access tab -> Connection...
    3. Select "Only the list below" and Add the DynDNS/Mailhop IP addresses or CIDR ranges you trust. Repeat in Relay... to limit relays.
    4. Leave your internal subnets (LAN, MTA) allowed so legitimate internal hosts still connect.

  • Host/firewall level enforcement

    • If the router cannot filter by source IP, enforce source restrictions on the server. On Server 2003 use IPsec policies (IP Security Policies MMC) to permit TCP/25 only from the Mailhop IPs and block others, or run a host firewall appliance that supports source-based ACLs. Test carefully — misconfigured IPsec can block admin access.

  • Practical troubleshooting steps

    • Enable SMTP logging (SMTP Virtual Server -> Properties -> General -> Enable logging). Default log folder is under %systemroot%\System32\LogFiles\SmtpSVC1; check timestamps and remote IPs against DynDNS Mailhop logs.

    • Capture traffic on the server (Microsoft Network Monitor or Wireshark) with a filter like:

      tcp.port == 25

      If DynDNS shows retries but no SYNs hit your server, the router/NAT is dropping or mis-translating.

    • From an external host test connectivity: telnet mail.yourdomain.com 25 — a 220 greeting indicates proper end‑to‑end reachability.

Notes and cautions

  • Confirm the current Mailhop IP ranges with DynDNS before locking them into rules; many relay services use pools that can change.
  • If the BEFSX41 continues to fail under load or for intermittent connections, replace it with a small-business router that supports robust NAT and source ACLs.

Recommended Answers

All 5 Replies

Member Avatar for malan_in

check the DNS server and MX recored, use traceroute and check the latency....

Member Avatar for DimaYasny

a good tool would be MRTG, especially if you leave it running overnight.

also see what happens if you try to send mail directly to your server - does dyndns lag in resending, or is that your server having problems after all.

Member Avatar for bobbyraw

has anyone in the company set up mail forwarding, for their account this can be a real bummer, by this i mean say i have an email yout your company, but i want to get my mails when i am at home ( you presently dont have web access) i would set my person email to grab mails from your server so i can access them through my personal mails when i am at home

Member Avatar for lsutiger

Thanx for all of the replies. I narrowed it down to our router's poor port forwarding ability. It is just a BEFSX41.

To try and curb the spam, I also was using 2525 as my external port for SMTP and 25 for my internal port. Once I changed it back to 25, the delays stopped. Is there any way I can setup the SMTP plugin only to accept connections from certain IPs? That would rock since we should only get connections from Dyndns' server.

BTW I am using WS2003, IIS 6.0 and the SMTP plugin from microsoft.

Member Avatar for DimaYasny

the easiest way is to open incoming port 25 fot the dynds IP(s) on the firewall

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