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Some of the things you need to stop Hotmail, Gmail, Yahoo!, AOL and other providers using server filtering with SpamAssassin or similar or having "blackhole" filtering (SpamHaus etc) from sending mail direct to junk are:
Correctly formatted headers for the From: and/or Sender: names.
The To: header being the actual recipient rather than "list member", "undisclosed recipients" or some other such generic entry with the actual recipient sent in the BCC field.
less than 50 names in the CC and/or BCC fields.
The sender IP having a reverse DNS lookup at the authoritative name server.
The "from" hostname having a SPF or TXT record in DNS.
Not all these are required, but at the very least rDNS and correct From: headers are essential
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