Port 25 is the standard email sending port (SMTP). Many ISP's block those to help prevent spamming from their services. Unfortunately, this causes a problem when you try to send email from your mail server, mail.yourdomain.com.
If you get an error that your mail server is not found/not responding, and everything else on your email client is configured properly most likely the problem is due to blocking on port 25 by your ISP.
If you are using Microsoft Windows:
1. Click the Start Menu button.
2. Click Run.
3. Type: cmd
4. Hit the Enter button. (this opens a command window)
5. Type: telnet yourdomain.com 25
6. Hit the Enter button.
If you are using Linux or MAC just open a console and type:
telnet yourdomain.com 25
and press the Enter button.
You should see a welcome message, which states that you are connected to your SMTP server. If you do not see such a message, then your ISP is blocking port 25. Our workaround to correct this issue is for you to use port 2525 or port 26 with our service OR Set your outgoing mail SMTP to your ISP provider. Instead of having your mail SMTP set up to mail.yourdomain.com, you can set it up as mail.isp.com