Following #19182 and experimenting with the Redmine Mail Handler, for use within my company, we ended up with the following need :

• In some cases, accept both known senders and unknown senders, but we did not want to allow AnonymousUser to be used, so we needed to override the sender with a specific user :

  --post-user SENDER
  

• In some cases, override all senders (known and unknown) with a specific user as specified above :

  --override-sender YES/NO
  

• Since we are overriding the sender, and we did not want to lose this information, we opted to allow the command-line rdm-mailhandler.rb script to take custom headers and footers for the imported mail body (to be added right after the cleanup process so as to minimize interference) :

  --email-header TEXT
  --email-footer TEXT
  

The patch included adds :

• the above options to extra/mail_handler/rdm-mailhandler.rb
• the code to process them within app/models/mail_handler.rb

Right now we are using these options in this fashion (this is used on top of the patch from #19182), in a script called as destination for e-mail coming from the outside :

/usr/local/share/redmine-2.5/extra/mail_handler/rdm-mailhandler.rb -v \
    --override-sender yes \
    --post-user yuubinkyoku@internal.domain.com \
    --email-header "Mail sent by : $SENDER" \
    --certificate-bundle /etc/ssl/certs/ca-certificates.crt \
    --url https://$URL/ \
    --key $API_KEY \
    --project $LOCAL

SENDER, LOCAL are variables set by Postfix upon calling the aforementioned script.
URL and API_KEY are derived from the DOMAIN variable and are not relevant to this example.