What is SMTP?
SMTP (Simple Mail Transfer Protocol) is the standard way email is sent from an app or service to a mail server and passed between mail servers.
Every email you send travels over SMTP. Your email app hands the message to your provider's SMTP server, which passes it on toward the recipient's mail server. Reading mail is a different job, handled by IMAP.
The settings you need
To send through a mailbox, an app needs the SMTP host name, a port, the security type, and a username and password. Port 587 with STARTTLS and port 465 with SSL/TLS are the common choices. The login is usually the same as for IMAP.
Sending through your own mailbox's SMTP server means replies come from your real address and are sent by your own provider, the same as if you had written them in your usual email app.
SMTP and inboxr
inboxr sends approved replies through the SMTP details you enter for each mailbox. If your outgoing settings match your incoming ones, an "outbox same as inbox" option copies them across. Replies are threaded under the original message, and each mailbox has send-rate caps to protect your sender reputation.