In the context of email marketing, a port number is a numerical identifier used to route network traffic to the correct application. When you configure an email server for sending or receiving emails, you must specify the correct port number to ensure that your emails are delivered successfully. Common ports for email services include port 25, 465, 587 for SMTP (sending emails), and ports 110, 995 for POP3 and 143, 993 for IMAP (receiving emails).