Mailing List Message #10120
From: Bill Cole <>
Subject: Re: 15 second delay when sending mail
Date: Wed, 30 Jan 2002 20:49:44 -0500
To: SIMS Discussions <>
At 11:29 PM +1100 1/30/02, Steven  imposed structure on a stream of electrons, yielding:
I have set up a SIMs server (1.8b9) and am using Cyberdog as my mail client. When i check for mail it is very fast. However, when i try sending a message, there is a delay of approximately 10-15 seconds before it is sent. When i use the same mail client to send mail via my ISPs mail server, it sends it without delay.

Therefore, it must have something to do with how my mail client is connecting to SIMS smtp to send messages. Is there any obvious configuration change i can make to fix this behaviour?

Not likely.

SIMS does some DNS sanity checking that other MTA's do not, and in some cases this can cause the sorts of delays you describe. In brief:

1. When a client connects and announces its identity (EHLO or HELO SMTP command) SIMS does a forward resolution on the name claimed by the client. If that name resolves to the client's IP address, SIMS will note this fact in the Received headers it adds to messages received on that session. If the lookup fails or points to some other address, SIMS will write Received headers that emphasize the IP address over the claimed name. This check cannot be turned off.

2. When a client issues a MAIL command with a sender address AND SIMS has "Verify Return Paths" on, it will attempt to resolve the domain part of the sender address for an MX record, and failing that, for an A record. If SIMS can find neither, it will reject the message. The details of the rejection (4xx or 5xx response) depend on the nature of the DNS failure. This can be turned off by turning off "Verify Return Paths"

Note that any DNS query which fails is very likely to take many seconds because a proper check will involve multiple DNS servers being queried. Since mail is going through, it's clear that the problem is not a bad sender address, but it could well be a sender address whose domain part has slow DNS. One of the most common curses of SIMS operators is the use of the kludgy old "MacDNS" nameserver that Apple briefly maintained. That nameserver has one advantage: it is free. It has many disadvantages, the most widely visible one being that it is about 2 orders of magnitude slower on the very fastest Macs than BIND on a 386-20 running SCO. If you are using MacDNS for domains that SIMS works with a lot, I promise you persistent and unavoidable delays.

A different possible cause, especially with Cyberdog and only if you are using the client on the same Mac as the server, is unrelated to DNS. Many network clients on the Mac (especially older ones) have very poor performance when doing 'loopback' connections. They mishandle their connections in such a way that starves any background processes (such as a SIMS server running on the same box) for time and especially for attention from OT. There have been versions of Eudora, Newswatcher, and Netscape with this trouble over the years, so it would not surprise me if Cyberdog (which never really matured as a product) had the same issue.
Bill Cole                        

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