Thread:User talk:Parent5446/Swift Mailer and VERP/reply (6)

In that case it makes even more sense to do it as a plugin. The plugin would have access to whatever context it needs, and whatever code is sending the mail does not need to worry about the HMAC calculation. In other words, you want a separation of concerns so that anybody who is using SwiftMailer only need to worry about what is actually being put in the message.