When I try to update IM() using email subsystem(forward reply to IM address) we get the error, that message exceeds 15000chars. but it has only couple of lines. But the email chain is growing.
Please,advice why system generates such errors (will send email which i've sent to IMas an attachment to this case).
You can turn on workflows logs ( active links/filters) to check which field is complaining about field length.
The workflows logs can tell you the exact field name and form name.