See @Gary post below.
https://forum.emclient.com/t/rules-applying-specific-account/60642/2
“When creating the Rule, or afterwards you can go back and modify an existing Rule, include processed with account in the argument”
“eM Client Rules documentation extract”
https://www.emclient.com/webdocumentation/en/10.0/emclient/default.htm#Rules/New%20rule.htm
“processed with account - this way you can set the account on which the rule should be applied, if it should be just one specific account and not all accounts”