Postfix and sending with multiple domains
I'm having trouble finding the correct documentation for some problems that one of our servers is having with sending mail. We host multiple clients using virtual hosts and we use postfix as a MTA to send web form information both to their users and to the client. Some larger email providers are blocking these emails because they think that it is spam. Some of the blocks may be blocking other people in the rest of the block of IPs (eg, 55.55.55.n) that we do not control. I think this is primarily because these email providers do not like emails from one domain being sent from another hostname.
Anyway, I am just looking for documentation/forum threads, etc. on how to send up sending services for multiple domains. I do not need to setup accounts on the server because this is not marked as the mail server in DNS (I can setup these accounts if it is necessary, but I'm looking to avoid these issues).
Any help would be greatly appreciated. Thanks in advance!