Symptoms

Customer is trying to change the connection strings for Email settings in the K2HostServer.exe.config file.
 

Diagnoses

This section of the config file gets encrypted when K2 Blackpearl Server starts up. There are ways to decrypt this section, however you would need to log a support ticket in order to get the utility that can do that.
 

Resolution

It is much easier to use the ConnectionStringEditor.exe program located in the K2 BlackpearlHost ServerBin directory.

Using this tool it is easy to change the email settings / accounts that have been configured during Blackpearl Configuration, and it's easy to add or remove accounts on the fly.

Please make sure that you click the Save button in the top left corner when done (before closing the program) and it's a good idea to schedule a restart of the K2 Blackpearl service as well.

I might add here that since 4.6.9, a lot of different Config files have been consolidated into the K2HostServer.exe.config file, please view this KB:
http://help.k2.com/kb001637