No, sorry, will not do that.
One, phones these days have lots of built-in "large" memory, 16 GB is the minimum it seems. Even if so it's 10 GB usable, that's still plenty, and I just won't believe that AquaMail takes up all that on its own.
Second, microSd storage is going to be less reliable, and I just do not want to deal with more cases of "the database got corrupted, I lost all my messages and accounts" (yes, they happen even with K9 Mail, there is no "magic").