Hey Adam,
Highly recommended by Nicholas - I hope that it will meet your expectations then
I don't know how RSMail is written, that's why without testing it with CMandrill I won't be able to tell you what the result would be.
There are 2 ways to use the mandrill service. One is smtp and the other one is the API.
Using smtp is the easiest option - just provide the smtp server in joomla's config and you are ready to roll!
The next option to use Mandrill with Joomal is - cmandrill. What we do is - we override the standard JMail class in Joomla. This way all extensions that use JMail to send their mails will use our class and we will send the mail through the API.
Sending the mails through the API normally has benefits such as speed + you can tag your content etc.
Now back to RSMail -> if they use the standard JMail class - you can use it with CMandrill. If they don't, they most probably have a configuration for smtp (or they use the global options in Joomla) you can go for mandrill through smtp.
With that said - mandrill also has limitations when you start for the first time. AT the beginning you cannot send a lot of e-mails, but with the time when you get some positive reputations the limit goes up.
If you go over the limit - the mails are queued and they will be delivered a little later.
With that said I love mandrill and right now I cannot think of a reason to use a different service. Our mails get delivered faster than the mails that people get from paypal on purchase
(although they are sent few seconds later when the APN call comes to our server). And since I use it - the number of e-mails I get about a non-delivered e-mail has gone way down. And even if I get such an e-mail - I just check the stats in mandrill and can see if the mail was sent or not...
Hope this information helps!
Cheers,
Daniel
P.S. We will be releasing a new CMandrill version begining of May. If you use the CMandrill on joomla 3.4 make sure that error reporting is set to none in your global config (there is a notice that gets triggered on j3.4 - nothing critical!)