how to change earlier sender mail address?

Dear Sir,
Please note that I have set windows mail for gmail.com and receiving and
sending is no difficulty.
However whenever dialog box is generated for a mail (picked up from any
excel data) then following message appears.
Quote
The message could not be sent because the server rejected the sender's
e-mail address. The sender's e-mail address was ''.

Subject 'trial'
Server Error: 421
Server Response: 421 4.1.0 Authentication Required - is not a
registered account. Please register at http://www.smtp.com to use this SMTP
(outgoing mail) server.
Server: 'SMTP.com'
Windows Live Mail Error ID: 0x800CCC78
Protocol: SMTP
Port: 25
Secure(SSL): No

Unquote

Suggest (How to delete earlier sender mail address and introduce new one?)

Regards
Manohar, ,

On Thu, 23 Jul 2009 18:25:01 -0700, Manohar Raut <Manohar
> wrote:

>Dear Sir,
>Please note that I have set windows mail for gmail.com and receiving and
>sending is no difficulty.
>However whenever dialog box is generated for a mail (picked up from any
>excel data) then following message appears.
>Quote
>The message could not be sent because the server rejected the sender's
>e-mail address. The sender's e-mail address was ''.
>
>Subject 'trial'
>Server Error: 421

421 - The service is not available and the connection will be closed.

>Server Response: 421 4.1.0 Authentication Required - is not a
>registered account. Please register at http://www.smtp.com to use this SMTP
>(outgoing mail) server.
>Server: 'SMTP.com'
>Windows Live Mail Error ID: 0x800CCC78
>Protocol: SMTP
>Port: 25
>Secure(SSL): No
>
>Unquote
>
>Suggest (How to delete earlier sender mail address and introduce new one?)
>
>Regards
>Manohar, ,

Authentication Required = password .