Building a Simple SMTP Client with PycURL involves using the PycURL library to interact with an SMTP server and send email messages.

Here is an example of how to use PycURL to send an email message:

import pycurl

c = pycurl.Curl()
c.setopt(c.URL, 'smtp://smtp.example.com')
c.setopt(c.USERNAME, '[email protected]')
c.setopt(c.PASSWORD, 'password')
c.setopt(c.MAIL_FROM, '[email protected]')
c.setopt(c.MAIL_RCPT, ['[email protected]', '[email protected]'])
c.setopt(c.UPLOAD, 1)
c.setopt(c.READFUNCTION, data.read)
c.perform()
c.close()

In this example, we first import the PycURL library and create a new Curl object. We then set the URL option to the SMTP server we want to connect to. We also set the USERNAME and PASSWORD options to the credentials of the account we want to use to send the email. The MAIL_FROM and MAIL_RCPT options are used to set the sender and recipients of the email. The UPLOAD option is set to 1 to indicate that we are sending data to the server and the READFUNCTION option is used to set a function that will be used to read the data to be sent. Finally, we call the perform() method to send the email and the close() method to close the connection to the server.

It’s important to note that this is a very basic example and in real world usage you will want to set some other options such as the mail_from and mail_rcpt to the actual email addresses, and also you will want to add the message content with the READFUNCTION option as well. Also, you can use a library such as email to create the message object, and then use it in the READFUNCTION option.

It’s also worth noting that PycURL is not a library that is widely used for sending emails and other libraries such as smtplib or yagmail are better suited for this task, since they provide a more pythonic interface for sending emails.