ISP (Internet Service Provider)
An Internet Service Provider (ISP) is an organisation that provides internet access services, and in the context of email marketing, the term is also used to refer specifically to the major consumer and business email providers, such as Gmail, Yahoo Mail, and Microsoft Outlook, that host millions of mailboxes and make inbox placement decisions. When email marketers talk about ISPs, they are usually talking about these inbox providers: the companies whose filtering systems decide whether your email lands in the inbox, the spam folder, or is rejected outright.
Each major ISP runs its own filtering algorithms, maintains its own reputation databases, and applies its own rules about what constitutes acceptable sending behaviour. The major ISPs that matter most for B2B email deliverability are Google (Gmail), Microsoft (Outlook.com and Exchange Online, which serves many corporate email environments), Apple (iCloud Mail), and Yahoo (Yahoo Mail). Each has different spam filtering logic, different authentication requirements, and different tools for senders to monitor their reputation within that provider’s system.
For B2B marketing teams, understanding ISP-specific behaviour helps explain why a send might perform well at Gmail and poorly at Outlook, or vice versa. Using ISP-specific postmaster tools and feedback loop services gives you direct visibility into how your sending reputation is perceived at each major provider. Monitoring performance separately by inbox provider domain, analysing engagement rates for Gmail contacts versus Outlook contacts, helps identify provider-specific issues before they become serious deliverability problems.
What is the difference between an ISP and an email service provider?
An Internet Service Provider (ISP) provides internet connectivity and, in the email context, refers to the organisations behind Gmail, Yahoo Mail, and Outlook.com that host recipient mailboxes. An Email Service Provider (ESP) is a platform that businesses use to send bulk marketing and transactional email to their customers, such as Spotler. ISPs are the recipients’ email infrastructure; ESPs are the senders’ marketing infrastructure.
Why do different ISPs have different spam filtering behaviour?
Each major inbox provider develops its own filtering algorithms trained on different datasets of reported spam and legitimate mail from their own user base. Gmail is known for prioritising engagement signals; Microsoft’s filters put significant weight on authentication and sender reputation; Yahoo emphasises complaint rates. Understanding these differences helps explain why a campaign might have different deliverability outcomes across providers and informs where to focus troubleshooting efforts.
What is Google Postmaster Tools and how does it help email senders?
Google Postmaster Tools is a free service from Google that provides senders with data about how their emails are performing at Gmail specifically. It shows your domain reputation, your IP reputation, spam complaint rates from Gmail users, delivery errors, and authentication pass rates. It is one of the most useful diagnostics tools available for B2B senders whose audience includes significant Gmail usage. Accessing it requires verifying ownership of your sending domain in the tool.