Handling Unsubscribes. It is also important to clarify the different ways that someone can, intentionally or unintentionally, unsubscribe from your list with a good ESP. Note whether they can click on the unsubscribe link, or click on the complaint button (if they are using a service like AOL). This may automatically unsubscribe them through the feedback loop, or the email can hard-bounce (i.e., the email address does not exist any more).
It is very important for these features to all be in place, because if these subscribers are not unsubscribed immediately and you send to them repeatedly, you run a much higher risk of being blocked by one or more Internet service providers (ISPs).
IP Addresses. It is also important to choose an ESP that either gives you a private Internet protocol (IP) address or has you share an IP address with a relatively small number of other customers. The IP address, whether shared or private, is the caller ID of your emails. If too many ESP customers share an IP address, you can easily run into problems with being blocked by ISPs for mistakes made by other customers of the ESP. Most reputable ESPs do their best to keep out the spammers, but even legitimate email marketers sometimes get their IP address blocked by an ISP—and it can take some time to get the IP address unblocked.
Contracts. Finally, if you are in the process of choosing an ESP, it is a good idea to choose one that does not require a contract term beyond one month. That way, if you are not happy with the service, you can cancel it and switch to a more suitable ESP. Not every ESP is going to be a good match for your needs and a good value for you.