When it comes to e-mail manipulation, RubyMail has always been my answer. The fortunate thing about RubyMail is that it parses each message into RMail::Message objects, which can also be converted back into e-mail with to_s.

This makes for simple modification of messages. The issue is slightly complicated by stuff like multiple From: fields. So, if you’re forwarding a message and simply want to modify header fields, be sure to delete the fields from the message first.

Here’s an excerpt from a simple mailing list script:

 msg.header.delete('From')
 msg.header.subject = "[#{ list_title }] #{ msg.header.subject }" 
 msg.header['From'] = "#{ from_recip } <#{ reply_to }>" 
 Net::SMTP.start( 'localhost' ) do |smtp|
     list['recipients'].each do |recip|
         to_email = "#{ recip['name'] } <#{ recip['email'] }>" 
         msg.header.delete('To')
         msg.header['To'] = to_email
         smtp.send_message( msg.to_s, msg.header['Reply-To'], to_email )
     end
 end

Note: This code won’t run from Irb unless you setup some variables and arrays. The point is to simply illustrate deletion of the header fields.