SMTP doesn't work

Hello,

I would like to send mails via smtp. I also tried some wp plugins to configure smtp but at least smtp could not connect to host.
I would like to send the mails via a remote service like gmail but couldn’t figure out how to get things running. I have access to the console as root and the question is what I have to configure on linux level. In yast I configured the mail to send mails to the provider but I don’t know how to check the different levels (linux, apache, php, wp) if they are configured correct or not.

Does someone knows a fool proof tutorial on how to get stmp started with linux and wp?

Thanks a lot,
Vanessa