Deliverability: Make sure the provider has a good reputation for delivering emails to the inbox, and not getting flagged as spam.
Scalability: Ensure that the provider can handle your current volume of emails, as well as any future growth.
Cost: Compare the pricing of different providers to ensure that you are getting a good deal for the volume of emails you plan to send.
Security: Look for a provider that has strong security measures in place to protect your email data.