If you want to allow your users to be able to use your SMTP server from outside you will want to require authentication (the "Only Authenticated SMTP connections may relay mail" setting).   This will require creating a mail password for each user.

I setup my email server a week ago it was working fine but just after 2

days I can see somebody is sending email through my server. I think they

are spammers, I turned off my server for several days and turned back

on just after 30 minutes the spammers back in and starts sending email

again. Can somebody help me configure/secure my mail server so that It

will not let spammers use it. Thanks and Cheers..

Go to Configuration / MercuryS, then on the 'Connection control' tab under 'Relaying control' ensure the first 2 check boxes are ticked. (These are called 'Do not permit SMTP relaying of non-local mail' and 'Use strict local relaying retrictions'.)

Doing this will stop spammers relaying through your server.  It may also prevent you sending mail out - depending on your configuration.  Ask again if this happens.