Recommendation for a Email Marketing Solution

Hello Guys!

I am starting a ProSites + WHMCS network. I would like to create an email autoresponder that would send emails automatically, depending on the Pro Sites level a user is currently on. I know this will require custom development, but would like your guidance here.

The idea is to have different email sequences ready, designed specially for each different level. To start, I will have 3 lists:

1. Free trial
2. Paid level 1
3. Paid level 2

The idea is to send emails with special promotions to motivate users to move to the next level. This way I can maximize profit.

The idea is to promote a discount only to users who have not yet paid and stuff like that.

I was hoping that you could point me in the right direction. Maybe I could use some of your plugins to start.

Thank you!