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Comment by etjossem | Hacker News Reader

SendGrid engineer here.

We have a similar challenge in working with the fixed number of IPv4 addresses we're able to get our hands on. We offer our customers dedicated IP addresses for outbound email, but there are sometimes situations where a customer doesn't want to hold onto one forever.

We want to make sure mailbox providers (like Gmail and Hotmail) have time to realize the IP is not in use, rather than mistakenly associating it with the old customer. It makes sense to hold onto it for a while after the customer releases it before allowing another customer to purchase it. We call this the "cooldown period."

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Gmail primarily uses domains for reputation, according to their docs (and in practice). I never had to deal with much IP management when sending to Gmail or G Suite recipients.

Microsoft does use the range of black holes, e.g. Spamhaus, including using their own, so etjossem is right about having to cool down a bad IP before requesting whitelisting. Almost all ESPs and black holes will reject whitelisting requests without a sufficient cool down period anyway.

Gmail is certainly ahead of its time. I haven't tried sending with IPv6 addresses but the Gmail documentation claims it will work.

Just from looking at my own mail servers and mail headers, Gmail will happily both send and receive mail over IPv6, and has for years. Same with Outlook. Oddly, despite Apple's push for IPv6 support, iCloud doesn't seem to have an IPv6-capable MX.

You're spot on about Google; they've moved forward from this approach and lean more heavily on domain-based information now. But they're a rare exception, and they have very little to go on when the domain is just "sendgrid.com" (i.e. the customer hasn't set up whitelabel yet). So we want to make sure we're always giving customers new or cooled IPs!

I'm surprised you willingly burn your own domain's reputation. Other email-as-a-service companies require a validated domain with SPF and DKIM in the DNS TXT records or use a burnable subdomain, e.g. freeaccount12345.sendgrid.com; or even better, 12345.sendgridfreeaccount.com.

Yep, we use burnable subdomains if you haven't whitelabeled.

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