When you add email in Microsoft Outlook or other email client software, SMTP port default is 25, POP3 port default is 110.
Some ISPs block port 25 to force users to use their email server.
In that case, you can configure your email client to connect SMTP via port 26.
Micosoft Outlook Instruction:
1. Open Microsoft Outlook 2007. From the Tools menu select Account Settings.
2. Select your account under the E-mail tab, then click Change button.
3. The Internet E-mail Settings dialog box appears, showing the settings for your existing account. Click the More Settings button.
4. Go to Advanced tab, under Outgoing server (SMTP), change port 25 to 26.
5. Click OK button to apply the change.
SMTP & POP3 server are mail.your-domain.com
For Microsoft Outlook 2010 the process is the same except instead of starting from the Tools option in the main menu you will start with the File tab in the main menu