This is a little different between Windows and Mac.   Any version of Windows: Press ⊞ Win+R , type cmd, then press ↵ Enter. Mac: In Finder, select “Applications,” then “Utilities.” Double-click the “Terminal” icon.  You can also access Terminal by typing it into Launchpad and clicking it. Type telnet mail.server.com 25 where "mail.server.com" is the name of the Simple Mail Transfer Protocol (SMTP) server of your email provider (such as smtp-server.austin.rr.com) and 25 is the port number used by the SMTP service.   You should receive a reply like "220 mail.server.com.” Port 25 is the port for most mail servers, but some network administrators move SMTP to a different port, like 465 (a secure port) or 587 (for Microsoft Outlook users) .  Ask your administrator (or check your account information) for the correct port. If you receive an error message, such as "Cannot connect to host on port 25," and you are sure that port 25 is the correct port, the mail server is likely experiencing an issue.

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Write a summary of the previous text in Vietnamese:
Mở cửa sổ terminal. Bắt đầu kết nối telnet.