Is it possible to use EPIMail with our own SMTP and not use the EPI service(with the montly payment)?
No, the EPiServer Mail product has been specifically made to be used with the EPiServer Mail service.
The advantages of this service instead of using just any SMTP server is high performance and offloading most of the work from your own server environment.
Does the payments is per month (300 $ for 50000 mails per month) or is it yearly payment (300 $ for yer for 50000 mails per month)?
The price is per month including 50 000 mails/month etc for the first step in the price ladder.