I’d like the option of being able to blacklist by the reply-to address.
You can normally do that via a Custom Rule
eg: Using eM Client V11 Beta, you can setup a custom rule for a “blacklist by reply-to found in header” as per the below example which hopefully will also work in eM Client V9 & V10.
Rules Documentation - https://www.emclient.com/webdocumentation/en/10.3/eMClient/Default.htm#Rules/New%20rule.htm
