The IP you are using is not authorized" generated by mercury/32 mail server
When I send emails to Gmail it generate this error but when I send mail to my ISP provided email, it is sent successfully.
What should I do?
I haven't used Mercury since the days of Netware 3, but I guess your problem is that you have to configure it for SMTP auth if you want to send mail via GMail, because of course GMail is not an open relay and not meant for submission from a server. There should be some kind of configuration option for that in Mercury.
Please note that GMail will rewrite the FROM address of all mails sent with it to the corresponding GMail account.
Why do you want to do this in the first place?