Yes, you can. Install your Mailplane license on any Mac you use.
Single-user license: Install on all Macs you personally use. Use it at home, school, work, etc. There's only one limitation: Make sure you're the only user. Please do not share your license with anyone else.
Family license: Allow up to five (5) family members living in the same household to use Mailplane on their Macs. As with the single user license, there is no machine limitation for any of the five users.
Site license: For a given number of users working in the same organization. Again, each user may use it anywhere.