BLOG

Delivery can be counter-intuitive

We all know that receiving ISPs rate limit incoming email. With the volumes of mail that they’re currently dealing with they must do that in order to keep their servers from falling over.

A client was dealing with rate limits recently. These were not typical rate limits, in that the recipient ISP was 4xxing mail. Instead, the recipient ISP was not accepting any incoming connections. The client was having a bit of a difficult time understanding what was happening and why the problem wouldn’t be solved by increasing the rate at which they were trying to send to the ISP.

Imagine if you will, that at every ISP there is a reception desk that manages the incoming calls. The receptionist is under orders from the to limit the number of calls coming in. When the phone rings, the receptionist can do any of the following:

1) answer the phone and put the call through (250 message accepted)
2) answer the phone and put the caller on hold (connection hangs or delivery is slow)
3) answer the phone and tell the caller to call back later (4xx message deferred)
4) fail to answer the phone (no connection at all)

The delay the client was seeing was #4, in that they were attempting so many connections at once that the ISP was just not answering.

In this case, reducing the number of connections attempted worked. The “receptionist” was not so overwhelmed by the number of ringing lines that she was able to actually answer all the calls and put them right through.

While lowering the rate at which the client was attempting to delivery seems counter intuitive to getting improved delivery, because we understood the mechanism we could lower rate and get an increase in delivery.

Comment:

Your email address will not be published. Required fields are marked *

  • HE.net DNS problems

    Hurricane Electric had a significant outage of their authoritative DNS servers this morning, causing them to return valid responses with no results for all(?) queries. This will have caused delivery problems for any mail going to domains using HE.net DNS - which will include some of their colocation customers, as well as users of their free services - but also will have caused reverse DNS to fail for most servers hosted by Hurricane Electric worldwide, so if any of your mail is being sent from HE hosted machines you may have seen problems. (We're HE customers so we noticed. Still happy with them as a vendor.)No Comments


  • 65.0.0.0/8 DNS issues

    If you're sending email from any address beginning with a 65 - in 65.0.0.0/8 - it's possible you'll see some delivery problems. Something appears to be broken with dnssec signatures for the reverse DNS zone, leading queries for reverse DNS to fail for anyone using a dnssec aware DNS resolver (which is almost everyone).1 Comment


  • Our green bar certificate is going away

    Later today we'll be switching from an Extended Validation ("green bar") SSL certificate to a Domain Validation certificate. This isn't exactly a planned change but I'm waiting for responses from Comodo before I go into it too much. I'll share some more details next week.3 Comments


Archives