Allow reply-to individual instead of topic/forum (mailing list feature)

Hello all,

I am trying to understand how to properly make this plugin. I’m currently stuck in my plugin.rb, as it seems that I cannot properly replace the MessageBuilder class.

Here is the current plugin.rb (and the repo in general):

https://github.com/tareko/replyto-individual/blob/master/plugin.rb

I have tried to understand how to do this by reading a few topics I could find in searches, but as of yet no luck:

When I run the plugin, I get the following, which I suspect reflects that the class is not properly replaced:

/vagrant/lib/email/message_builder.rb:20: warning: already initialized constant Email::MessageBuilder::REPLY_TO_AUTO_GENERATED_HEADER_KEY
/vagrant/lib/email/message_builder.rb:20: warning: previous definition of REPLY_TO_AUTO_GENERATED_HEADER_KEY was here
/vagrant/lib/email/message_builder.rb:21: warning: already initialized constant Email::MessageBuilder::REPLY_TO_AUTO_GENERATED_HEADER_VALUE
/vagrant/lib/email/message_builder.rb:21: warning: previous definition of REPLY_TO_AUTO_GENERATED_HEADER_VALUE was here

Any thoughts or advice?