SMTP configuration for WSS3 & windows server 2008

Question

I have Windows server 2008 & WSS3 installed .. when I try to add user and send mail to him I got this error "The user or users have been added successfully, but there was an error in sending the e-mail message. The server may not be set up correctly to send e-mail." ... I have a problem in sending mails from sharepoint to different servers  ... I have made the following configuration

1. Start IIS6 Manager (Start/Adminisdtrator Tools)
   2. Drill down to the SMTP Server and right-click for the Properties.
   3. On the Access tab - Authentication button, have 'Anonymous' as the only one selected
   4. On the Access tab - Connection button, select 'All except the list below' and leave the list blank.
   5. On the Access tab - Relay button, select 'Only the list below' and add two entries - the ip address of the current server and the loopback adapter (127.0.0.1)
   6. On the Delivery tab - Outbound Security button, select 'Basic Authentication' and include the username and password of company's mail /or GMail account.
   7. On the Delivery tab - Outbound Security button, tick the TLS option.
   8. On the Delivery tab - Advanced button, set the 'Smart host' to "mail.company.com"/or "smtp.gmail.com".

9. From IIS7, In leave SMTP Mail empty, and select the second option.
10. I configured the sharepoint Outgoing mail from central administration.
11. Also, I tried both 25 and 587 as the port number.("Delivery" tab-->"outbound connections" --> TCP Port)
12.Restart IIS.. and then I tested again but no mails sent from sharepoint.

I used the "SMTP Test Tool" and tested my company's mail, it send successfully.

Also I installed a sharepoint feature that send mail(using System.Net.Mail), it doesn't throw exceptions but doesn't send mails.

Any help or advice would be highly appreciated!
Thanks in advance,
.

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Amira