Here is the ultimate guide or points to keep in mind while buying an SMTP server for email marketing management from providers.

Go for SMTP servers that come accompanied with a hosting plan. It ensures that all domain restrictions on sending emails are taken care of.
Buy the service that the pricing structure for the SMTP server is within a reasonable smtp server plan.
Check for the capacity they offer you with the plan at what speed and milestone to send emails in a day.
Check for significant features like selective disperse, real-time delivery reports, etc.
The reputation of an SMTP service provider is an important point you should take into consideration.