sending an email from ANY computer

Question

hi,
I want to send an email from my app. I obviously don't know who is about to use this app. how can i send an email considering i don't know what SmtpServer to use?
yet, i do knoW the following:
the computer has outlook, dunno whether it's matter or not
the computer is in my country (maybe i need to try all the possible smpts?)

thanks a lot :)

p.s.
my users probably don't know what server they're using >_<