can gnus support fake mail address?

[winsphinx]

for an example:
...............
(setq user-mail-address "xxx@example.com")
(setq smtpmail-auth-credentials
    '(("smtp.163.com"   25    "USERNAME"   "PASSWORD")))
(setq smtpmail-default-smtp-server "smtp.163.com")
(setq smtpmail-smtp-server "smtp.163.com")
(setq message-send-mail-function 'smtpmail-send-it)
...................

in this case, i use a fake mail address (setq user-mail-address
"xxx@example.com") , and now it fails on sending mail when smtp server
authenticate it. on the other hand, if i use my true address, i worry
about when i use newsgroup, i will get many spams.
anyone can provide a good way?
thanks for any help.