How do you send emails? Without dealing with IP bad reputation?

Hello hackers:

I hope you can help my with the problem I'm facing

I'm currently working on a project. My business requires to send an email to my customers so they can download the generated images they paid for.

I chose Sendgrid (Email API) it worked fine for the first 5 emails tests I did, but now any new email is stuck in "processing" state. Reading the log in Sendgrid it seems that the IP has bad reputation and the email is being rejected by Hotmail servers (the fake customer I'm testing with uses Hotmail)

Sendgrid has a plan that offers IPs with good scoring but it cost $89/month.
I'm starting my project so I'm not in a position to purchase that plan yet.

HOW DO YOU DO IT?

• What email delivery system do you use to send emails to your customers?
• Does exist a service that charges per email sent instead of a monthly basis?
• Any other alternative to Sendgrid with better IP reputation?

Thanks very much for the advices