Send email via localhost using PHP

Papercut - https://github.com/ChangemakerStudios/Papercut-SMTP/releases
PHPmailer - https://github.com/PHPMailer/PHPMailer/releases/tag/v6.5.0
The problem
If you ever send emails from an application or web site during development, you're familiar with the fear of an email being released into the wild. Are you positive none of the 'test' emails are addressed to colleagues or worse, customers? Of course, you can set up and maintain a test email server for development -- but that's a chore. Plus, the delay when you are waiting to view new test emails can radically slow your development cycle.

Papercut SMTP to the rescue!
Papercut SMTP is a 2-in-1 quick email viewer AND built-in SMTP server (designed to receive messages only). Papercut SMTP doesn't enforce any restrictions how you prepare your email, but it allows you to view the whole email-chilada: body, html, headers, attachment down right down to the naughty raw encoded bits. Papercut can be configured to run on startup and sit quietly (minimized in the tray) only providing a notification when a new message has arrived.

Papercut SMTP Background Service
Papercut SMTP has an optional "always on" service to receive emails even when the client is not running. It's installed by default with Papercut.Setup.exe -- but can also be installed separately by downloading Papercut.Service.zip, unzipping and following the service installation instructions.

It is configured to use localhost:25 for the mail server.
The error message says that it can't connect to localhost:25.
Therefore you have two options:
Install / Properly configure an SMTP server on localhost port 25
Change the configuration to point to some other SMTP server that you can connect to

You can use other mail services, not only Gmail. Later, when you have your own server, install your mail server.

A mail server (or email server) is a computer system that sends and receives email. In many cases, web servers and mail servers are combined in a single machine. However, large ISPs and public email services (such as Gmail and Hotmail) may use dedicated hardware for sending and receiving email.

In order for a computer system to function as a mail server, it must include mail server software. This software allows the system administrator to create and manage email accounts for any domains hosted on the server. For example, if the server hosts the domain name "techterms.com," it can provide email accounts ending in "@techterms.com."

Mail servers send and receive email using standard email protocols. For example, the SMTP protocol sends messages and handles outgoing mail requests. The IMAP and POP3 protocols receive messages and are used to process incoming mail. When you log on to a mail server using a webmail interface or email client, these protocols handle all the connections behind the scenes.

Mail server software is available for multiple platforms. The most popular mail server for Windows is Microsoft Exchange Server, an enterprise product used by large businesses. However, many other options exist, including Ipswitch IMail Server, IceWarp Mail Server, MailEnable, and hMailServer. Popular Linux options include Exim for sending mail and Dovecot and Courier for receiving mail.

We can say that an email server (or mail server) is your digital postal service. It’s a machine or application responsible for handling messages. In other words, an email server receives and delivers emails, this is its function.

So, when you send an email, your message usually goes through a series of email servers until it reaches the recipient.

The process is so fast and efficient that it looks simple, but there is a great deal of complexity behind sending and receiving emails.

To avoid confusion, it is important to be clear that the term email server can have different meanings depending on the context. Sometimes an email server can mean a computer or a machine that has a complete system that includes different services or applications.

At other times, the term email server can be used precisely as a synonym for some of these services or applications.

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