What are incoming vs outgoing mail servers?

Outgoing mail servers use SMTP (Simple Mail Transfer Protocol) to send messages from senders to recipients. These are the servers your ESP operates—they accept your campaign and transmit it across the internet. Incoming mail servers use IMAP or POP3 to let recipients retrieve messages from their mailboxes. Gmail's incoming servers store your subscribers' mail; your ESP's outgoing servers delivered it there.

The naming reflects direction of flow. Your email client connects to an outgoing SMTP server to send mail, then connects to incoming IMAP/POP3 servers to receive mail. For marketers, the outgoing infrastructure matters most—that's where deliverability lives. You never interact with recipients' incoming servers directly; you hand off to their SMTP gateways, which then route to storage.

Understanding this distinction clarifies where problems occur. If your email "doesn't send," it's an outgoing server issue (check ESP status, authentication). If subscribers "can't see" your emails, they may have arrived but be filtered or in spam—that's the incoming side's doing. Most deliverability work focuses on making outgoing servers look trustworthy so incoming servers welcome your messages.