How do ESPs connect to mailbox providers?

ESPs connect via **SMTP** (Simple Mail Transfer Protocol), the standard for email transmission. They maintain sending infrastructure: IP addresses, domain authentication, and server capacity to deliver messages to **MBP** receiving servers. No special integration required for basic delivery.

ESPs optimize connections through: proper **DNS** configuration, **IP warmup** procedures, throttling to respect **MBP** rate limits, retry logic for temporary failures, and bounce processing. Better ESPs maintain relationships with **MBP** postmaster teams for issue resolution.

Some ESPs participate in special programs. **Gmail**'s **Postmaster Tools** integrations, **Microsoft**'s deliverability partnerships, and certification programs like **CSA** provide enhanced visibility or preferential treatment. These relationships represent **ESP** investment in delivery quality.

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