Evercontact has a service called ContactRescue which can update all of your Salesforce contacts/leads overnight by analyzing your and your sales teams' email from a period of 1-5 years back. 

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• What benefit can I expect? An average ContactRescue analyzes 12,500 emails and yields 250 new or updated contacts
• How many emails accounts can ConstactRescue analyze at a time? ContactRescue scans every email account linked with multi-account feature for Gmail users or every email account received in your Outlook and analyzed by Evercontact.
• How long does it take? This depends on the number of emails to be analyzed - from a few hours, to 24h or even up to 4 or 5 days for ContactRescue + (5 year or 60,000 emails).