Better you can do E-Mail marketing
Talking Spam, It all depends on what content you send the customer via E-Mail. If Completely promotional then your mail send to the Spam box of the customer.
Talking to Email Spam, there are so many e-mail marketing soft. available, you have to research, other wise take experts helps who regularly do the e-mail marketing. If done in improper manner then your mail count as Spam using some E-mail marketing soft.
If you wish to forward the mail via G-mail, how much? Your account might get restricted from the Big G!