The error comes up when sending out email from System5. In this case, we are emailing directly from System5 but shouldn't be any different from emailing through an Email Client.

The error simply means that the default SMTP (Outgoing Mail Server) port “25” is being blocked and cannot proceed sending out.

The quickest way out of it, is to use the Alternate SMTP Port Number (587).

Format: SMTP Mail Server:587

You can apply the same on the email client if you get the error.