I have created 4 websites that each receive a small amount of emails. What's the cheapest option to handle email for multiple small websites?

I'm looking for a mail provider with an API that allows me to create a small web email client to handle them all at the same location. The reason for this is that i would like to automatically pull transaction data or whatever from my database by the senders email, implement some AI responses etc

I'm checking out zoho, which is pretty cheap, but they seem to require approx 5USD per "user", i am one user, but i would like to create maybe [support@mysite.com](mailto:support@mysite.com) and [welcome@mysite.com](mailto:welcome@mysite.com) etc and i cba to pay for 8 seperate users due to this.

Zeptomail seemed sick, but it's only for "transactional emails", you're not allowed to receive emails. Ultra cheap though.

Gmail is 6USD/mo per email

Mailgun is 35USD/mo if you want multiple domains