Yes, you can ensure that IMail requires 128bit or higher cipher strength using the following steps:
Open the IMail SSL Configuration Utility. This utility is located in the IMail Program group under the Start Menu.
Select the "Options" tab and enable the "Force Clients to use 128bit SSL" option.
Click "Apply" and then close the SSL Configuration Utility.
Stop and Restart any IMail services that use SSL.
Your IMail server should now only allow 128bit cipher strengths.