I am running IMail 8.15, and when I ran 8.15 on W2K Advanced Server with a much slower machine (Athlon 3200+ with 1GB RAM) I did not get the NTDLL.dll SMTPD32.exe failures when running the verify and revdns connection filters, but after upgrading to a much faster Athlon X2 6400+ with 4GB RAM running on a Windows 2003 Server (x64) I have these limitations with these filters.

My theories (yes, I have more than one) ...

1. The new faster system is processing more data than the SMTPD32.exe can handle, particularly when thses filters are enabled, causing some sort of buffer issue.

2. Imail 8.15 is not as stable with Windows 2003 Server, particularly, the x64 bit version.

Incidentally, the verify and revdns connection filters induced crashes occur even when the server is running at off peak times.  If I check all three, even when the server is under light load (off peak) the SMTP service will crash, in a very short time.  If I check just one, it will take longer, but eventually crash.

A side effect to this problem is that the number of smd files in the spool becomes quite large.

As a side note, I have considered these issues to also be related to DNS performance.  I have also experimented with running a private secondary DNS server on the mail server itself to reduce I/O with the Primary DNS server.  This DNS server allows recursion using it's root servers (which I have confirmed to be all up-to-date) and pulls the hosted domain information from the primary DNS server.  Also, since my public primary and secondary DNS servers are set to not permit recursion IMail does not work well with them, since IMail requires a recursive DNS server to resolve all the mail going in and out and through the server.

Something else that had no effect on the smtp / connection filter crash issue is my recent DNS implementation of SPF (v=spf1 a mx ?all), simply by adding a text entry.  This has had a great impact on reducing "backscatter" that comes from spammers spoofing my customers valid email addresses to pass certain validation filters.  This has reduced the load on my mail server, but had not impact on the issue of this post.

IMail 8.15 Admin