How do I support multiple ISP accounts in the Postfix SMTP client (smarthost) for relaying email? For example:
[a] email@example.com must be relay via smtp.isp1.com.
[b] firstname.lastname@example.org must be relay via smtp.isp2.com.
[c] email@example.com must relay all my default email via smtp.mycorp.com.
My ISP requires that mail from my dynamic IP to our small business email addresses uses their outgoing SMTP servers. This is probably done to reduce abuse and spam but now I’m not able to send email and local Postfix log file displays authentication failure message. How do I relay mail through my mail ISP servers using Postfix SMTP under Linux / UNIX like operating systems?