I've defined a workflow in SWF implementing the workers in Java code and it's working as expected. However, I'd like to be able to start an instance of it by sending a mail. What are the good practices to do it? Is there any example published? Any help is welcome.
Don't have an example for you, but you can receive incoming emails with AWS SES, SES can then be setup to invoke a Lambda function upon receipt and once running in Lambda you can pretty much do anything you want - including starting an SWF worker.
http://docs.aws.amazon.com/ses/latest/DeveloperGuide/receiving-email-receipt-rules.html
Creating Receipt Rules for Amazon SES Email Receiving
A receipt rule enables you to specify what you want Amazon SES to do with mail it receives for one or more recipients or domains. The receipt rule consists of a condition and an ordered list of actions. If the recipient to which the incoming mail is addressed matches a recipient specified in the condition, then Amazon SES performs the actions specified in the receipt rule. For more information about the role of receipt rules in the email-receiving process, see Email-Receiving Concepts.