How to write this HTML format? _html/css_WEB-ITnose

Recently I wrote a C email sending program based on SMTP protocol in Linux environment. The content sent is:
Welcome to register: Click to register
The sending is successful and you can send it to your email The display is still:
Welcome to register: Click to register
It feels like there is no conversion, just string display. Dashenmen, is the HTML format of the content I sent wrong, or how to convert and send the HTML format?

Reply to the discussion (solution)

The style is normal, check to see if he has been asked elsewhere, post it and take a look!

Where can you choose the language of the outgoing mail server? Whether to send it in html format or plain text. Select html format and it will be displayed in html mode.

Where can you choose the language of the sending mail server? Is it sent in html format or plain text? Select html format and it will be displayed in html format

Yes, that’s the problem

Where can you choose the html format when sending the email server language? Is it sending in plain text? If you select html format, it will be displayed in html format

Hello, when I write in C language according to the SMTP protocol, how do I choose to send in html format? Thank you