I'm going to go out on a limb here and say Microsoft's Graph API will be the best solution for connecting to services provided through Office 365. What would happen if AquaMail decided to not provide connectivity using Graph API? In that case, unless the user was seeking only an email solution, I feel as though subscribers to Office 365 would best look to applications other than AquaMail to meet their needs.
How can we find out if Graph API is on AquaMail's development roadmap?