Archive for January, 2010
Undeliverable mail
Jan 19th
One of the most common email related support calls I get is someone wondering why they’re receiving “Undeliverable Mail Returned to Sender” notifications when sending out email to a particular email address. These “bounce back” messages will tell you why the email couldn’t be delivered, but often times the language used isn’t immediately understandable by someone who isn’t an IT professional, or “into computer stuff”. I thought I’d go over some of the more common undeliverable errors, and explain what they mean. I made a test email address for this purpose, blogtest@iphouse.com – and then sent several emails to it that were intended to be bounced for various reasons. Here’s the first bounce back message:
This is the mail system at host smtpin-2.iphouse.net. I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.
<blogtest@iphouse.com>: permission denied. Command output: maildrop: maildir over quota.
Anti-spam Part 2, Bayesian Spam Filtering
Jan 5th
Well, Andrew and I kinda stepped on each others toes last month, but I’ll go into a little more depth on some of the things he touched on. Last month I talked about the frontend of our anti-spam filtering via Greylisting.
At the opposite end of our anti-spam system is content filtering. We use a third party vendor for this, MailFoundry in the form of two appliances. An appliance is a machine that you plug in, and is suppose to work with minimal configuration.
Now the MailFoundry appliances are “black box” systems. We don’t know how they work exactly, but we’re pretty sure that one of the techniques they use is Bayesian spam filtering.
Bayesian spam filtering uses the concept of probability to evaluate each token in a message, assign a weight to each, give the overall message a rating based on this weight, and evaluate the message based on a preset threshold.
Ok, unless you’re up on your statistics or logic based calculus, or a computer nerd with Wikipedia handy, I know your eyes just glazed over. Rest assured, you are not alone.
