Mail App is using internet connection to capacity (Retina Macbook Pro Early 2013) My internet connection is working to capacity while "Mail" is open. Of course, it was loading all my mails when I used it the first time, but now all mails are on the machine and still "MagicanPaster" shows me that the internet connection is busy all the time while Mail is open.
The settings are searching for new mails every 5 minutes. I am using OS.X 10.8.3.

Maybe my internet connection is just to slow, but is 400 KB/s for "Mail" not just too much? What options do I have to decrease the traffic?

Update: Even if I change the settings to "Searching for new mails every 15 minutes" the connenction is working still to capacity.
Is it possible to set priorities for internet connection, so that I can use skype or facetime without lacking and still have mail in the background? Is anybody else experiencing the same problem?
Mail > Preferences > Accounts > Advanced

There is an option called "Keep copies of messages for offline viewing"
If you have that set to All messages and their attachments, then Mail will download your entire Gmail archive, which could be several GB.
I suggest setting that to All messages but omit attachments, then quitting Mail, then deleting ~/Library/Mail/V2 and relaunching Mail.  It will take several minutes to clean up and re-download all the messages, but this time without attachments.