SMTP is the protocol used to send email. Your email program or sending tool hands a message to your provider's SMTP server, which passes it on to the recipient's mail server, also over SMTP.
The settings you'll see
- Host: the provider's SMTP server, such as
smtp.gmail.com. - Port: 587 (encrypted with STARTTLS) or 465 (encrypted from the start). Port 25 is for server-to-server traffic and is usually blocked for apps.
- Login: the mailbox address and its password, an app password, or a sign-in token.
SMTP and cold email
A cold email tool sends through each connected mailbox's SMTP server, so emails go out from the real mailbox, with its own reputation. Reading replies uses a different protocol, IMAP, or the provider's own API. That's why connecting a mailbox from most providers asks for both SMTP and IMAP settings.
Common problems
Wrong port, the provider having switched off password sign-in (Google and Microsoft both prefer sign-in tokens), or a mailbox blocked for sending too much in a day.