What are the best services for sending newsletter/marketing emails?

MailGet has an inbuilt feature known as SMTP Routing that permits you to integrate with multiple SMTPs relay for sending emails as per your requirement.
SMTP Routing will let you send the email to those 1,000 subscribers from any of the email sending server as per your need. In other words, you can manually select the emails (in %) to send from any of the particular SMTP relay based on your requirement.