If you have forgotten your Original Startup password and cannot reset the password or disable the startup password protection, then you will have to “Delete the eM Client Settings.dat file” as per @Gary post below from the following thread.
https://forum.emclient.com/t/how-to-reset-the-startup-password-on-em-client-7/60427/2
“There is a file you can delete, but you will lose most of your settings. You won’t lose any data, nor items like Rules, signatures and templates”.
"Close eM Client, and then delete C:\Users_[username]_\AppData\Roaming\eM Client\settings.dat. You will have to show hidden items in Explorer to do that.
If you have a Mac, then the eM Client Settings.dat file is in the following hidden folder - “Users\yourusername\Library\Application Support\eM Client”