First good luck for new business. I am gonna write some tips here to start work as ESP( email service provider), hope you will find helpful
Backend software - Powermta , Postfix
As we all know powermta is costly so you can start with postfix if you dont have budget to buy powermta although powermta is best smtp server software , All major esps like mailchimp , sendgrid use powermta as backend, so you can go with any open source smtp server software
Frontend script-
Interspire , oempro , Active campaign and any custom
You can choose any front end script as per your requirement and budget
Server requirement -
I suggest you to go with atleast 8 GB Ram , Dual quad core processor , 500 + 500 ( raid) GB hard disk + 256 ips . This configuration is enough to send more than 1m mails per day
Why need 256 ips ?
There is no limit per ip to send mails its totally depends on ips reputation. but i suggest you to start with 256 ips because if someone else will do spam in your pool then it could also effect your ips reputation. its not necessary to use all ips to send mails , you can reserve them for future
configuration and sending tips -
1- ensure all authentication mode configured properly
Dkim , spf , sender id , dmarc ( domainkey is not needed now so dont need to care about domainkey)
2- Ensure your server is not operating as an open relay , its really important
3- Configure your server so it does not send DSNs for non-existent addresses
4- ensure smtp server configured with proper ISP , bounce and backoff rules, you have to be sure you are not exceeding volume and connection per ip according to ESPs requirement
5- Send slowly to all ISPs by default. Increase the rates you send if you detect you are hitting your max-msg-rate. Do not try to blast out email unless you absolutely have to
6- Allow only send to optin subscribers
7- Offer clear and easy unsubscribes on all your emails, you could get blacklist if you will not take care of unsubscribers and it also harm your reputation
8- Ensure you're adding Message-ID and Date headers to each email, some spam filters (like SpamAssassin) can add points for not having these.
9- register for feedback loop ( its really very very important ) do not send mails without feedback loop
10- FRDNS should match
Feel free to post here if you want to know anything else
