Resending an mbox file of messages

I'd like to read in an mbox file and "resend" the message to a
different SMTP address. The following code seems to work for plain
text messages but not for MIME messages.

from =
to =
mail = tmail.to_s
Net::SMTP.start(SMTP_server, 25) do |smtp|
      smtp.send_message mail, from, to

Seems like this should be pretty straight forward since I don't want
to modify the message body at all.

Any ideas what I should be doing differently for MIME messages?