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301 redirect to new domain likely wouldnt help, you're bringing the tainted link juice with you, assuming links is the issue. Most 'experts' think you wont recover from Penguin until the next Penguin update. Its unclear to me if this agency 'kept sending automated links' that you would have been cleared by any update if this process was still ongoing. Bear in mind Penguin isnt just about links. Changing domain might be a valid option but it does depend on the site itself, authority, number of spammy links you still have etc. Not that its much comfort but id also say, remember Google isnt the only source of traffic.