Mail uses all my memory

Hi All

I use a late 2012 iMac with 3.4ghz i7 and 16gb of memory.

Recently I updated to Sierra and eversince I have times where my Mac seems to freeze randomly. I now discovered, that Apple Mail seems to cause this. It randomly starts to use all my RAM within seconds and uses over 18gb of memory (parts as swap space) which cases the freeze of all other apps and even the mouse cursor.

As soon as I quit the mail app, everything recovers and goes back to normal. While using my Mac for regular work, it uses about 4gb of memory, mail eats all the rest randomly!

I think it is a bug since it happens without any traceable cause (no large attachements or so... even if I just launch mail and do nothing else, it happens...).

Would be happy to see a Workaround or an upgrade!

Thanks!