Core differences
Function separates them most clearly. SMTP pushes a message from a client to a server, and between servers, until it reaches the recipient's mail system. IMAP pulls messages from a mailbox to a client and keeps the two in step.
Their roles follow from that. SMTP is the outbox path and ends the moment a receiving server accepts the message. IMAP is the inbox path and begins after delivery has already happened.
State handling is the difference with practical consequences. SMTP is stateless with respect to the message once transmitted, so it has no concept of read, unread, filed or deleted. IMAP maintains exactly that state on the server, which is why the same mailbox looks identical on a phone and a laptop.
Ports differ accordingly. SMTP submission uses 587 with STARTTLS or 465 with implicit TLS. IMAP uses 993 with implicit TLS, or 143 with STARTTLS.
A mail client needs both configured. One set of settings sends, the other receives, and a client with only outgoing settings will send successfully while showing an empty inbox.
POP3 as the alternative to IMAP
POP3 is the older retrieval protocol, in RFC 1939, and it downloads messages rather than synchronising them.
The distinction matters when more than one device is involved. POP3 traditionally removes messages from the server after download, so a message read on a laptop may never appear on a phone. IMAP leaves the authoritative copy on the server and mirrors it everywhere.
POP3 uses port 995 with implicit TLS, or 110 with STARTTLS. It remains reasonable for a single-device setup that wants local copies, and it is the wrong choice for anything else.
Server settings in a mail client
Configuration is two blocks. The outgoing block takes the SMTP host, port and security mode, and its incoming counterpart takes the IMAP or POP3 equivalents.
Provider hostnames follow a convention rather than a rule: smtp.provider.com for sending, imap.provider.com for retrieval. The values come from the provider's own documentation, since they vary and a guessed hostname fails at connection.
Authentication is required on both, and most major providers now issue application-specific passwords rather than accepting an account password from a third-party client.
Why this distinction matters for sending applications
An application that sends email needs SMTP or an HTTP API, and needs IMAP for nothing at all. Confusion between the two produces integrations that request mailbox access for a system that only ever transmits.
The exception is a system that must read replies, such as a support tool threading conversations or a sequence tool detecting responses. That is genuinely an IMAP or provider-API job, and it is a different product from sending.
We keep a fuller walkthrough of how the two protocols pair up in a mail client in our IMAP and SMTP settings guide, because most configuration mistakes come from mixing up which of the two a given integration actually needs.
Worth stating the boundary plainly for this topic: Nitrosend does not do inbound mail. There is no receiving, no parsing, and no mailbox read or write. Reading a mailbox means Gmail's API, Microsoft Graph, or IMAP directly.