Depending on your uses one might be cheaper than the other, fastmail is pretty cheap if you only have 1 user for all the domains (you can alias whatever email addresses you want) ($40/year personal plan)
Rackspace I haven't really used so I don't know how they handle multiple domains it might be cheaper to use them with multiple users. They do require a minimum $10/month
I use fastmail https://www.fastmail.fm
Depending on your uses one might be cheaper than the other, fastmail is pretty cheap if you only have 1 user for all the domains (you can alias whatever email addresses you want) ($40/year personal plan)
Rackspace I haven't really used so I don't know how they handle multiple domains it might be cheaper to use them with multiple users. They do require a minimum $10/month