It can be much harder to send emails today than it was in the past. Spam has become a major problem, and every provider is taking actions to curb it. Unfortunately, many of these actions can also make it much more difficult to send legitimate emails.
Many residential ISPs don't allow outgoing connections on port 25. If your residential provider blocks port 25, then you won't be able to establish an SMTP connection. In this case, you may consider renting a virtual private server to run this chapter's code.
Even if your ISP does allow outgoing connections on port 25, many SMTP servers won't accept mail from a residential IP address. Of the servers that do, many will send those emails straight into a spam folder.
For example, if you attempt to deliver an email to Gmail, you may get a response similar to the following:
550-5.7.1 [192.0.2.67] The IP you're using to send mail is not authorized 550-5.7.1 to send email directly to our servers. Please use the SMTP 550-5.7.1 relay...