Why is (Yosemite) Mac Mail hitting my internet so hard? Mail is downloading 250-500Kb/sec and has been for a while now. I can't figure out what it's doing. Is there some way to find out? I tried Window->Activity but it didn't show anything being downloaded.
How about using opensnoop to try to work out what Mail is doing? opensnoop is one of the tools that make up the Dtrace toolkit. It is part of the OS.

sudo opensnoop | grep Mail


That command should show you what files Mail is opening. That might point you in the right direction to what it is doing.