SMTP Mail Adapter settings

Hi Experts,

I have small doubt regarding SMTP Mail Adapter settings.

Right now i am sending files in attachment to the Users through SMTP.

But now User needs in MIME type for which i can see settings in Security Parameter.

I can see is S/MIME, when i flag that i can see Duration like Days, hours, minutes, seconds.

Could you please tell me what is the significance of it.

Appreciate for your suggestions

Regards,

Bhasker