- What would you recommend I do?
Do you have firing authority?
In all seriousness, this IP needs to go out of circulation.
The advantages (accountability/non-repudiation) of sending DKIM-signed mail via Marketo is in turn a disadvantage if somebody who legitimately represents your company sends spam (or in a spammy pattern). There's no quick fix and your sends will likely be negatively weighted for some time, since Google is smart enough to recognize that you have signed spam in the past.