I see what you mean now; thanks for the additional information.
I don't see why what we are doing would affect the DKIM signature. The signature your server adds would be based on certain headers that your server added when you sent the message. Anything that Runbox adds after that (and what we may or may not choose to add) doesn't affect the signature your server has added as we are not modifying the existing headers the signature was based on.
That said, it's an interesting question and if you want to try it out with us then drop us a line at Support and I'll set something up so we can confirm that the signature is still valid.