The long tail of domains


I frequently get clients telling me that they have about 15 (20, 30) major domains on their list, and then a long tail of domains with only a couple of recipients. If you sort simply by the left hand side of the @, that’s true.
When you’re sending email, it’s not just the domain in the email address that is important. Of equal importance is the MX. The MX is what actually handles the mail and where many filters are applied. Sorting by MX, instead of simply recipient domain, can identify that most of your small business clients are hosted at a particular provider. The number of subscribers behind that filter may be enough to push that filter into your top 10 or even top 5 recipient domains.
There’s a much smaller tail when grouping recipients by MX domain. It makes it much easier to understand where blocks are happening. I have even seen cases where clients didn’t realize they were blocked at a commercial provider because they only saw the “onesie twosie” domains as undeliverable. They missed a real problem with blocking because they were looking at the wrong data.
I sometimes get the side eye from some ISP folks if I use the term receiver (because, well, they’re senders as much as they are receivers). But I use receiver to help distinguish between the recipient domain and the actual domain handling the email.
When was the last time you looked at your delivery by filter or MX rather than by recipient domain? What did you find?

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  • I never sort by MX but yes i did take that in consideration.
    It can be easily done with few lines script.

  • This is probably a silly question, but what exactly are you grouping on for the MX records — are you grouping by the root domain of the highest priority entry, or something else? Thanks for the post!

  • Not a silly question at all. You can’t just use the root domain of the highest priority MX because the root domain isn’t always the relevant one. We developed grouping rules manually based on what we were seeing in the database.
    A good example is MX. Looking at the MX records with a root domain of we see:

    • aspmx* are google hosted domains which have one set of filters
    • is actually corporate which has a different set of filters
    • gmail-smtp* is gmail which has yet a 3rd set of filters

    So we group the aspmx records together, but leave the gmail* and records alone.
    Another example is There are a lot of different filters contained in root domain, and grouping them together doesn’t give you a clear enough picture of what domains look like.

  • A lot of small domains may use a particular filtering service. For example, McAfee SaaS is, which is used by a lot of small businesses.

By laura

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