Setup and configure SMTP properly.
Provide quality and unique configuration for each customer.
Major ISPs like gMail , yahoo , hotmail , outlook have their limitations to recieve emails. ( eMails / ip / hr )
IP has its ows history and reputation. Warm up IPs can index in major ISPs.
This is THE Main thing for mass emailing. Select the best provider for your IPv4 IPs to gain high inbox rate and longlast.
