A lot of users ask us whether or not they should use a dedicated IP. Unfortunately, the answer isn't always straightforward, and the real answer is that it depends.
The primary benefit of using a dedicated IP is that the reputation of that IP is based only on the mail you are sending. Shared IPs, on the other hand, have many users sending, so their reputation is based on all of the email from many users. A dedicated IP also means that your mail comes from a specific IP. When using shared IPs, the IP used will rotate and not always be the same. Poor list management, unsolicited emails, or unengaged recipients will cause issues on both shared and dedicated IPs—a dedicated IP is never guaranteed to solve delivery problems.
If you are not sending a lot of email, a dedicated IP can hurt your deliverability because receiving servers are not seeing enough email to know whether to trust the IP or not. So each time you send an email, they are re-evaluating every aspect of the email as though it's a new IP rather than one that is established and reliably sending email.
In our experience, you should be sending at least 5000 emails per day, at least three days per week to justify a dedicated IP. A single dedicated IP can handle up to 500,000 emails per day (distributed throughout the day). Some ISPs limit how much mail they will accept from individual IPs, per hour or day. Because of this, if your list is concentrated in certain ISPs, the amount of email you can send from a single IP may be lower.
If you are sending to a lot of corporate domains that need to be able to whitelist the IP you are sending from, a dedicated IP can be a really good option. It makes it easier for your clients or users with those corporate domains to whitelist just your IP instead of all of Mandrill's shared IPs.
If you are planning to send more in the future, beginning the warmup process for a dedicated IP can be beneficial. Just be sure that you do not spend time warming up the IP, and then leave it dormant for awhile; consistency in sending behavior is beneficial when it comes to warming up IPs. Mandrill has an automated warmup option that you can enable when you request the IP (or any time after the IP is provisioned).