Add support for BCC

[delbertooo]

As mailhog is primarily a debugging tool we want to validate the correctness of bcc and cc as well.

See #98, #261

Possible solutions

• multiply the emails for each recipient
• show cc and bcc addresses in the preview

Duplicating emails won't help debugging and may make it even worse. So we went for the "show in preview" option.

Implementation

Bcc should not be contained in the mail source. Otherwise it wouldn't be a "blind" copy as the recipients could see the blind copy recipients. Every recipient (to, cc, bcc) should be part of the RCPT though. So from my understanding and the insight of this Mailtrap article bcc should be allRecipients - to - cc. This calculated bcc is what I implemented and added to the Message struct.

On the GUI side I decided to follow the suggestion of
jankonas and show cc and bcc alongside with "from" and "subject". Cc and bcc are only shown if they have content. This way the user can easily debug who should've received the mail.

[delbertooo]

I built a docker image as a preview with the cc/bcc support added by this pull requests. You can find it at delbertooo/mailhog-bcc.

[msde]

I needed exactly this yesterday, and the supplied docker image worked as expected.

[w455im]

Hello, can this be merged please ? it is very much needed 😞

[JoeFowlerJDi]

Another +1 for merging this, or else doing something else like it.

The current behaviour of ignoring bcc can very easily be mistaken for a bug on the mail sender's side. I had to do a little digging to realise that we were setting the bcc correctly, it just doesn't show up in mailhog.

[konigprinz]

Please merge this. As a developer, this is a very useful feature.