Is there an SPF record for mydomain.com?
Without more details it's hard to know what else to suggest. Can you give us an example of a message that is marked as spam? If you reveal the full headers for the message on the receiving end, does it include a header that indicates why it was identified as spam?
You might also add an A record for mydomain.com like this:
Code:
@ IN SOA mydomain.com root.mydomain.com. (
stuff
)
IN NS ns.mydomain.com,
IN NS ns2.mydomain,com.
IN MX 0 mail.mydomain.com.
IN TXT "v=spf1 a:mail.mydomain.com ~all"
IN A 10.10.10.10
mail IN A 10.10.10.10
IN TXT "v=spf1 a:mail.mydomain.com ~all"
etc.
Adding DKIM signing is also a good idea. You can install the opendkim package from the repositories and add the appropriate support in Postfix.
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