Email has two separate jobs: sending messages and retrieving them from a mailbox. A backend in your project has to be able to send mail but does not read mailboxes; that is why the Email (SMTP) connection asks for SMTP credentials only.
SMTP for sending
SMTP stands for Simple Mail Transfer Protocol and transports messages from the sending system to the recipient’s mail server. Your backend signs in to your provider’s relay with a username and password and hands over the message for delivery. For the Port field, 587 with STARTTLS is the usual value for authenticated sending; 465 with TLS is older but still widespread. Port 25 serves unauthenticated traffic between mail servers and does not fit an account used by an application.
The Mailtrap connection does not need SMTP because the platform sends through the API token and Sending API there. You configure another provider’s HTTP API in your application’s code, not in the SMTP connection.
IMAP and POP3 for mailboxes
IMAP keeps a mailbox on the server in sync across several devices, and POP3 downloads messages and optionally deletes them from the server. Both belong in mail clients or in a service for incoming mail; the platform therefore does not ask for them. If your application needs to read inboxes, you set up an IMAP account or your provider’s REST API in the project outside the connection.