He gave me some tips that I could pass on to our readers. So here you go:

• Comcast really, really hates it when you send to an old list (with lots of undeliverable email addresses). If you’ve got too many emails on your list that no longer exist, Comcast will block your IP address. This is a very strong argument for cleaning old, inactive members from your lists. Yes, I know it feels safer to keep old members on the list, because the number of recipients looks so much bigger. Yes, I know your boss keeps telling you not to clean your list, because they don’t want to lose any prospects. But those bad addresses are just holding back your overall deliverability and keeping the good addresses on your list from getting your message. Yes, I have numbers to prove it. In a later post.
• Comcast doesn’t like it if you send too many emails all at once to them (this one’s kind of a no-brainer). Here’s what makes them different: they’ll actually TELL you how many you’re allowed to send at once.
• As you can see from this chart, the rate of emails you’re allowed to send to Comcast at once depends on your IP’s “reputation.”
• Your IP reputation is based on at least two factors: Authentication (a technology standard that’s used to prove an email is not a forgery), and the IP’s “Sender Score” (Your IP’s Sender Score is kind of like a credit rating, and it’s the folks at ReturnPath who issue the score). MailChimp customers – Authentication is free with every MailChimp account, and through our relationship with ReturnPath we monitor our senderscore closely.

For those of you familiar with ISPs and abuse desks in general, none of this is all that new. The interesting stuff is how forthcoming and helpful Comcast is with their error codes and rate limiting charts. Overall, their new Postmaster site is one of the best I’ve ever seen (look at all the handy RSS feeds):

http://postmaster.comcast.net/

If you’re new to all this, and wonder how this applies to you:

• Don’t send to old email addresses. You’ll get yourself (and other innocent senders) blocked.
• At some ISPs, old email addresses get turned into spam traps. You send an email to a spamtrap, and you are instantly blocked. We had a case of a MailChimp user who sent to a 10-yr old email list (totally opt-in, and the guy luckily had IP/timestamps on all subscribers) and one of the addresses had been turned into a spam trap. The impact on his deliverability from a spamtrap hit was instantaneous and dramatic. Keeping his proof of opt-in for so long saved his behind.
• If you’re sending your emails from an ESP (like MailChimp, Constant Contact, etc) that has shared IP ranges, then one bad apple can truly spoil the whole IP range. So it’s important to go with an ESP that closely monitors their deliverability, and punishes bad senders (it’s for the greater good). Quality over quantity.
• Don’t export your entire email address book and assume they all want to receive email newsletters from you. Address book dumps are full of old email addresses, and are one of the most common reasons I see senders get blocked. Other reasons include: Fish bowls, sharing lists, and buying lists.
• If you have an old list of people who opted-in to receive emails from you, but you haven’t sent them an email in a really long time (or ever), then you need to delete any emails older than 1yr, then send the remaining subscribers a “remember me?” welcome email. Here’s a good example.
• If you send emails on a regular basis, and you think your system is cleaning the list for you, you still might want to segment your list by actives vs. inactives.

Comcast isn’t the only ISP that hates it when you send to old/dead lists.  AOL’s postmaster talks about IP reputation and undeliverables here. ReturnPath, the experts on email deliverability, mentions undeliverables here and offers some tips. Laura Atkins from WordtotheWise talks about “Letting Go” of old members.

In general, there’s a shift (again) in the way ISPs and email receivers judge emails as spam or not. Instead of relying on blacklists or focusing on bad content, they’re looking at your overall reputation as a sender. And list cleanliness (undeliverables, hard bounces, spam complaints) are huge factors in determining how clean your list is.

According to George Bilbrey, AOL, Gmail, and Yahoo have implemented DKIM email authentication.

For those of you who don’t know, authentication is a way to prevent email forgeries, and it can improve your deliverability. The AOTA says Authentication has hit its “tipping point.”

Since 2004, when email authentication started to really pick up steam, ISPs have been testing (and changing) their support for authentication. Some use one method for inbound, and another for outbound. Some support all methods of authentication. Some have abandoned it, or are in limbo. It can be confusing.

So we compiled a chart of which ISPs are using which authentication methods over here.

Of course, we’ve also made authentication “MailChimp Easy:” Our customers can authenticate their email campaigns with one simple click, and we cover all the major authentication standards (DKIM, Domain Keys, SenderID, and SPF).