When I try to send email I get the error "550 You must check your mail from this IP or SMTP Auth before sending to xxxxx@xxxxx.c

This error is usually seen when SMTP authentication is not enabled in the mail client.

To enable SMTP authentication in Outlook Express/2000:

1. Select the 'Tools' menu and click 'Accounts'
2. Highlight your mail account and click 'Properties'
3. Click the 'Servers' tab at the top of the window
4. Check the box labeled 'My server requires authentication' and apply the changes

To enable SMTP authentication in Outlook 2002/2003:

1. Select the 'Tools' menu and click 'Email Accounts'
2. Leave 'View or change existing email accounts' selected and click 'Next'
3. Highlight your mail account and click 'Change'
4. Click the 'More Settings' button
5. Click the 'Outgoing Server' tab
6. Check the box labeled 'My outgoing server (SMTP) requires authentication'
7. Leave 'Use same settings as my incoming mail server' selected and apply the changes

To enable SMTP authentication in Mozilla Thunderbird 1.x:

1. Select the 'Tools' menu and click 'Account Settings'
2. Select 'Outgoing Server (SMTP)' on the left
3. Check the box labeled 'Use name and password' and apply the changes