IMAP vs SMTP
In short: IMAP is how email is received and synced from a server, and SMTP is how email is sent. They are two halves of the same job.
IMAP (Internet Message Access Protocol) lets a mail client read messages that live on the server and keeps them in sync across your devices. When you open an inbox on your phone and laptop and see the same mail, that is IMAP at work. (POP3 is an older alternative that typically downloads and removes mail instead of syncing it.)
SMTP (Simple Mail Transfer Protocol) is the standard for sending and relaying outgoing email between servers. Whenever you hit send, SMTP carries the message toward the recipient’s mail server.
How it relates to MailFellow
To connect a mailbox that is not Gmail or Outlook, MailFellow uses IMAP to read incoming email and SMTP to send your replies. That means it works with virtually any email provider — you just supply your IMAP and SMTP settings.
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