The reason why you cannot send emails using your own SMTP Server is because your Internet Service Provider (ISP) is blocking Port 25. Port 25 is widely abused by malware to spread worms and spam. As a result, many ISPs are restricting its use. A preferred port for mail submission that we have implemented on all our servers is Port 587.
If you are using the following ISP:
Here's a procedure that you must follow in setting up your email client (MS Outlook, Thunderbird, etc). Assuming that you already setup an email account. Let us take MS Outlook 2003/2007 for an example:
1. Open Microsoft Outlook 2007
2. Click "Tools" menu, then "Account Settings."
3. On the "Email" tab, double-click the account name.
4. In "Server Information," put the following: Incoming mail server: mail.yourdomainname.com; Outgoing mail server (SMTP): mail.yourdomainname.com
5. Click "More Settings"
6. In "Internet E-mail Settings" window, click "Outgoing Server" tab.
7. Tick "My outgoing server (SMTP) requires authentication", with "Use same settings as my incoming mail server" selected.
8. Lastly, click "Advanced" tab, then change "Outgoing server (SMTP)" from port 25 to port 587
* Note: If you encounter any certificate error, approve it and you will be all set.