SMTP has been around for many years as a standard method of transmitting email from one mail server to another. Historically port 25 was the only game in town and so it was the only option for anyone using SMTP.
Over the years, spam, often caused by viruses that would infect a mail system, sending out hundreds and thousands of unwanted emails, became a problem.
To address this and other issues, other port options such as 587 have been introduced with enhanced authentication (STARTTLS). These new options have become more universally available through email services in preference to the older port 25 which typically has no security.
Today, in an effort to curb spam, most residential ISP's and cloud hosting suppliers block email from port 25 which is why you should avoid using this port.
If your SMTP gateway is configured with port 25 (no security), then you can either;
checkout our list of SMTP configurations for commonly used email services to see if your email service is listed and if there is another port option you can use or
contact your email service directly to see if they support other port options.