Postfix Configure Multiple ISP Client SMTP Authentication

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How do I support multiple ISP accounts in the Postfix SMTP client (smarthost) for relaying email? For example:

[a] [email protected] must be relay via smtp.isp1.com.
[b] [email protected] must be relay via smtp.isp2.com.
[c] [email protected] must relay all my default email via smtp.mycorp.com.