What is an ISP in email context?

In email, ISP (Internet Service Provider) usually means two different things, and knowing which one matters for your sending context is important.

First, there's the network ISP. That's companies like Comcast, Verizon, AT&T, or BT (British Telecom). They provide the internet connection itself. Your email travels over their network infrastructure, but they don't host your inbox. They're the pipes, not the destination.

Second, there's the mailbox provider ISP. In email's early days, companies like AOL, Yahoo, and CompuServe provided both the internet connection AND the email inbox. You'd dial up to AOL's network, and your email address was @aol.com. Those companies were true ISPs in both senses. Today, that dual role is rare. Most people get internet from one company (Comcast) and email from another (Gmail).

When email senders talk about ISPs, they're almost always talking about mailbox providers. Gmail, Outlook, Yahoo, iCloud. These are the companies that host inboxes, run spam filters, and decide whether your email lands in the inbox or spam. That's what matters for deliverability. The network ISP (Comcast, Verizon) doesn't make inbox placement decisions.

And the confusion comes from older terminology that stuck around. In the 2000s, AOL was an ISP that also ran one of the biggest mailbox systems, so "ISP" and "mailbox provider" meant the same thing. Now they don't, but the old language persists. If someone says "Gmail is an ISP," they mean Gmail is a mailbox provider. Technically not accurate, but everyone knows what they mean.

For your sending: focus on mailbox providers, not network ISPs. Authentication (SPF, DKIM, DMARC), IP reputation, engagement rates, spam complaints, those all matter at the mailbox provider level (Gmail, Outlook). Your reader's network ISP (Comcast, Verizon) doesn't factor into inbox placement decisions. If you're troubleshooting deliverability, you're troubleshooting with mailbox providers, not network ISPs.