E-Mail Deliverability : SPF and SenderID

First, the links:

http://www.openspf.org/

This relies entirely on DNS being set up correctly. You must have your MX records correct, you must have reverse lookups working right (ie: pointed to your mail server).

It's all about the text records.

Here might be an example forward zone for your domain, which has a few dns servers, a mail server, and a web server:

$ORIGIN mydomain.com.
@ IN SOA dns1.mydomain.com. primary.mydomain.com. (
2008010900 ; serial
1800 ; refresh
3600 ; retry
604800 ; expire
86400 ; minimum
)
; DNS SERVERS
mydomain.com. 300 IN NS dns2.mydomain.com.
mydomain.com. 300 IN NS dns3.mydomain.com.
mydomain.com. 300 IN NS dns4.mydomain.com.

;MX RECORDS
mydomain.com. 300 IN MX 5 mail.mydomain.com.
mail.mydomain.com. 300 IN MX 5 mail.mydomain.com.

;A RECORDS
mail.mydomain.com. 300 IN A 10.1.1.10
dns1.mydomain.com. 300 IN A 192.168.1.2
mydomain.com 300 IN A 10.1.1.2
;CNAME RECORDS
primary.mydomain.com. 300 IN CNAME dns1.mydomain.com.
www.mydomain.com. 300 IN CNAME mydomain.com.

; TEXT RECORDS
mail.mydomain.com. 300 IN TXT "v=spf1 a -all"
mydomain.com. 300 IN TXT "v=spf1 ipv4:10.1.1.10 mx -all"

...continued...