If you can't send email (but you can receive, in some cases) your ISP, firewall/antivirus software, or your router may be blocking the outgoing mail (SMTP) port 25. This is the standard port for sending mail, but it is often blocked because spammers commonly use this port to send mail from compromised computers.
The problem can often be resolved by changing the SMTP port to 587. To change the outgoing mail port in Microsoft Outlook, and Mac Mail:
- Click the "File" tab, then click the "Account Settings" button.
- Highlight the account you want to change, then click "Change" to open the settings window.
- Click the "More Settings" button, then choose the "Outgoing Server" tab.
- Check that "My outgoing server (SMTP) requires authentication" is chosen. If it is not, click the box.
- Check that "Use same settings as my incoming mail server" is chosen. If it is not, click the box.
- Choose the "Advanced" tab.
- Change "Outgoing server (SMTP)" port from 25 to 587.
- Click "OK".
- Click "Next".
- Click "Finish".
- Open "Preferences" in the "Mail" menu.
- Click the "Accounts" tab to locate your domain's email account. Open it.
- Open the "Outgoing Mail Server" window.
- Change the number in the "Port" field to 587.
- Check the "Use Authentication" box.
- Click "Continue".
- Click "Continue" again. Close any remaining open windows.