Holds total number of bytes user downloaded in this session.

A Long value.  

object.BytesDownload [= value]

The BytesDownload Property syntax has these parts:

From the time user connects to your server, up to time you're accessing this property, it will keep (and regularly increase) total number of bytes downloaded by connected user. This value can be used for some statistical or similar purposes.

You can reset this value to 0 at any time, or set it to any other value at any time. For instance, if you want to memory total downloaded bytes across all the sessions by the user, you should store this value to your database when user disconnects, and then take it back from database and put to this property when user logs in.

By its specification, SMTP protocol does not expect too many bytes to be downloaded by the user. Much more data will be received from user (message bodies).