B2B spam filters and domain blocking: Who blocked my mail?

Let's compare B2C versus B2B deliverability issues, shall we?

In the B2C (business-to-consumer) or DTC (direct-to-consumer) email marketing universe, the number of mailbox providers, while broad, is very heavily concentrated in the US. There are six providers, that when you add up their subscriber reach, they comprise over 90% of almost any typical US B2C email list: Google, Yahoo, Microsoft, Comcast and Apple. Anything beyond the top five is pretty much in the long tail. (That doesn't mean you ignore deliverability issues for smaller providers, but when you do a cost/benefit analysis of time investment versus reward, it is often clear that fixing an issue with a big provider first will yield more benefit.)

In the US B2B (business to business) email universe, things are not as obviously concentrated. There are more providers and it's not just a set of six controlling just about all of the mailboxes you'll want to send mail to. And to add to that, it's not immediately clear who hosts mail for a given domain name. There is a way to identify which ISP (internet service provider) or MBP (mailbox provider), though, and if you don't already know about it, it's something important that you should learn: the MX record lookup.

An MX record is a DNS record for a domain name that indicates which servers handle mail (MX meaning "mail exchange") for a given domain name. It is often a direct link that tells you which ISP or mailbox provider hosts mail for a given domain name. If you're wondering what to do when mail to a certain company gets rejected, knowing what service provider hosts mail for that domain can be very helpful.

Let's use my own domain wombatmail.com as an example. This link leads to my DNS lookup tool with wombatmail.com pre-filled in. Click on through, and you can see that the server names returned all end in google.com or googlemail.com. This means that mail for the domain name wombatmail.com is handled by Google (Gmail for Business).

Understanding which ISP or MBP hosts the mail for a given domain name doesn't necessarily tell you how to address a blocking issue with that ISP or company. But it is an important bit of insight that can help guide you on what to search for next. If you were having deliverability problems -- maybe spam folder delivery -- when sending to a domain hosted by Google, that very strongly implies that you have a Gmail deliverability issue and your next steps should be to investigate how to address Gmail deliverability issues, and maybe even to make a submission to Gmail's sender contact form.

So, when you find that you're blocked when sending mail to a given company, look up the MX record for that company's domain, then search online for information on how to request unblocking for that ISP or mailbox provider. Search for an unblock form or postmaster page or deliverability help page for starters, and go from there. There won't always be clear guidance for everyone, but you will find success for some, and I'm hoping you'll find it helpful.

Here are the domain MX record mappings for the top dozen US B2B email providers, by my reckoning:

I've added links to more info, unblocking, or postmaster pages where possible. I hope you find this information useful!

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