About mail servers

Simply put, a mail server sends and receives email. When ruby@protonmail.com emails klaasje@gmail.com, Protonmail’s mail server’s MTA sends the email to Google’s mail server’s MTA. Google’s MTA then passes the email on to Google’s MDA, which is responsible for storing the email. MDAs also run IMAP or POP3 servers so you can read your emails and send emails with an email client. IMAP and POP3 are protocols for email retrieval over TCP/IP. For sending email, you would use SMTP.