Did you ever get an answer to this question? I am in the same boat, with numerous assets--all only listed by IP address under AssetName--reporting as Mail servers, even though the majority do not currently show up in our DHCP address list as active leases. A couple that do show up at one location are Sonos sound system devices and one is an iPad. I am still evaluating Lansweeper, so I don't know if assets once identified this way are removed when they no longer show up, or if they hang around perpetually.
I have just begun to research this to find out why things get this designation but the response to your query being "ask tech support" doesn't encourage me that this info will be easy to find.
