Constant Contact also has an API if you want to do the integration yourself. The SugarCRM side of the integration is a little tricky though.
I would only recommend trying to do the integration yourself if you want to allow Constant Contact to change your Contact, Lead, Account and/or Target information. Which it a nice feature of Constant Contact, because the recipients can change their own contact information or a person can ask to be added to a mailing list, in which case you would want a target record to be added to SugarCRM, possibly.
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