Hello Shannon,
At the moment you can not choose which SMTP server will be used, there is already a logic implemented which is choosing the right SMTP server according to it's load.
However, I agree that it could a great feature to have better control over which SMTP server will be used. So if you want functionality like this available in the feature version of Kentico, you can suggest it and also other users will be able to vote for it (more users = higher priority), on the special page: Kentico UserVoice
Best regards, Martin