IMAP and SMTP
The two protocols a mail program uses: IMAP to read mail on the server, SMTP to send it.
IMAP (port 993 over TLS, or 143 with STARTTLS) lets a mail program read mailboxes that stay on the server. Every device sees the same folders and the same read state, which is why it replaced POP3, where each device downloaded its own copy.
SMTP is how mail is sent. Between servers it runs on port 25. From a mail program to its own server it uses port 465 (TLS from the start) or 587 (STARTTLS), with a username and password; this is called submission. Hosting providers commonly block outbound port 25 on new servers to keep their addresses off blocklists, while 465 and 587 to your own provider stay open.
Modern mail programs find the right servers and ports from DNS before asking the user: SRV records for IMAP and submission, or the autoconfig and autodiscover hosts. If those exist, a user only types an address and a password.