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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