Log in as a System Administrator, go to the gear at the top 3rd from left, then Security on the left side. The first tab is for IDS Blocks.
Oh my bad, I missed on your comment mentioning SM16. SmarterMail 16 does not allow configuration of this value via the web interface. I believe these values are adjusted at the root of your IIS site for SmarterMail, web.config should have these values.
SmarterMail 17 does allow configuration of the webmail IDS blocks.