I'm wondering if a few DNS experts out there could give me some input on SPF record configuration. Our company sends out about 50k - 100k emails a day, and most emails are on behalf of customers to their end users at various domains (no, we're not spammers, these are email notifications the end users have requested to receive). Some customers insist on making the FROM address use their domain name, but the emails leave our mail servers on our domain. SPF seems to be the way we could possibly avoid more spam filters, and delivery rate is very important to our company. The server configuration consists of a mail server that sends outbound only, out of a specific IP with proper MX, A, and PTR records. This is a sample of the SPF configuration I believe would be correct: Our company (example.com) records: IN MX 10 mail.example.com mail IN A <ip address> example.com IN TXT "v=spf1 mx -all" example.com IN SPF "v=spf1 mx -all" mail IN TXT "v=spf1 a -all" mail IN SPF "v=spf1 a -all" customer.com IN TXT "v=spf1 include:example.com -all" customer.com IN SPF "v=spf1 include:example.com -all" Our customer's (customer.com) records: IN MX 10 mail.customer.com mail IN A <ip address> customer.com IN TXT "v=spf1 mx -all" customer.com IN SPF "v=spf1 mx -all" mail IN TXT "v=spf1 a -all" mail IN SPF "v=spf1 a -all" customer.com IN TXT "v=spf1 include:example.com -all" customer.com IN SPF "v=spf1 include:example.com -all" I derived this from this tutorial: http://www.zytrax.com/books/dns/ch9/spf.html . The other part of this that may be of importance would be the NATing. The FQDN that the world sees for the outside address of the NAT is not the same as the inside FQDN that Postfix is using internally. Does this cause any problems with SPF? Any comments or suggestions would be great. Thanks in advance! Jeffrey