Have 3 servers - web server, mail server and outbound-only email server. The outbound-only email server sends transaction based emails on behalf of the web server (password reset, contact us, etc.)
How should I set up the DNS to improve email deliverability for the outbound-only email server.
www & * point to IP address of web server
mail points to IP address of mail server
Is an A record needed for the IP address of the outbound-only email server? If so, what should the prefix be?
mx record points to the IP address of the mail server
Is an MX record required for the outbound-only email server? (I don't believe so but asking for completeness)
SPF record (TXT) lists the outbound-only email server IP. DKIM is also a future possibility.
Per the answers below but summarized for future readers,
1) NO need for an A record for the outbound mail server
2) NO need for an MX record for the outbound mail server
3) YES for an SPF TXT record for the outbound mail server
4) Also need a PTR record for the outbound mail server. This is a good link for understanding PTR.