Redmine

I would like to use the truncate incoming emails settings with regular expressions. But I am not clear on where to place the regex.
Does it go on the settings incoming emails tab like this?

Or do I put it in code somewhere?

Thanks
Eric

Environment:
Unbuntu 16.04

Redmine version                3.4.6.stable
  Ruby version                   2.3.1-p112 (2016-04-26) [x86_64-linux-gnu]
  Rails version                  4.2.8
  Environment                    production
  Database adapter               SQLServer
SCM:
  Git                            2.7.4
  Filesystem                     
Redmine plugins:
  redmine_ldap_sync              2.1.1.devel