We just migrated on SmarterMail 15 as we had great comments about this software.
We were also ready to purchase the extended version as our company is bigger from the Free version account limits.
We noticed there is a core problem on the usability of "Drafts".
When you save a draft and later on you edit it again and you reply the email, the software does not show you the blue arrow that marks the message as "Replied".
By this problem you cannot understand which messages are replied, as it is with the normal procedure.
We hope the issue to be solved as soon as possible. Else we cannot continue using this product as this is a core usability problem.