How much email marketing is safe? Is it considered as SPAMMING?

Hello,
I have been using SEO for product promotion from many years. It?s effective and result oriented.But recently my one of client demand me to work for email mass marketing. I have searched and collected a list a top email marketing providers.
I have a question to ask: how much email marketing is safe? Is it considered as SPAMMING?
So far I know sending unsolicited email is considered as SPAM.

Please provide me some tips and service providers.
I would really appreciate all replies.
Thanks for giving valuable time

See original post by ShivJaiswaL