What are the affected AsyncOS version?I don't see anyone mention AsyncOS version whenever they post problems. Usually, different AsyncOS version may have different outcome thus always a good practice to mention it.
Pardon me. I believe M-Series runs same hardware as X-series, not C-series.Also, the M-Series apps comes default pre-loaded and it's not by activation via feature keys.
Alternatively, by having multiple MX records (each points to different IronPort X-series box) with the same weightage, it would become sort of load balanced.