To get right to your question, the answer is usually. It varies by email provider. If you delete an email from the handset, the email is then removed from the email server. Since most people read emails on their computer by visiting the email server directly or use an email retrieval program (like Outlook), they are no longer there.
You can download programs in the Market that alter the way you can interact with email accounts and also give additional options. Let me know if that answers your question. You can always test it out to see how your particular email setup reacts when you delete an email.
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