If you are mass mail sender or you want to send 1 million emails per day or more. Then google smtp, smtp server hotmail, aol mail server, yahoo email server and free smtp hosting server wont work for you.
Because gmail, aol, hotmail, yahoo and all free smtp server providers do not allow to relay unlimited emails per day or per hour from their email servers. They apply limits on outgoing emails which will slow down your email campaigns and you will loose the opportunity.
Thats why you need a dedicated smtp email service with multiple dedicated ips and make sure your server setup properly as per compliance with inbox landing with your domain name (i.e spf, dkim, dmarc, rDNS) and your smtp mail server IPs are warmed up and not blacklisted in any spam trapper.
To start sending mass emails you also need an bulk email software which allows you to manage your email lists, campaigns, suppression lists, and give you full tracking features. And to configure your mass email software to start sending emails you need a SMTP server address, username, password and a smtp port number to relay your email campaigns.