[dovecot] Document SPF and DMARC DNS records.

SPF DNS records specify which IP addresses may send email on behalf of
the domain. DMARC DNS records specify what happens when mail cannot be
delivered.

diff --git a/dovecot/README b/dovecot/README
index 8466c001a8a72de8407c5a99f76e3ae67eda08d7..fb6087f63396744622337999bc52b5a37fbf1ff5 100644 (file)
--- a/dovecot/README
+++ b/dovecot/README
@@ -49,10 +49,12 @@ Add DNS records. If no MX record is present, mail servers fall back to the A
 record (see https://serverfault.com/a/470651).
 
 ```
-example.com.      0 A       <ipv4_address>
-example.com.      0 AAAA    <ipv6_address>
-example.com.      0 MX    0 mail.example.com
-mail.example.com. 0 CNAME   example.com.
+example.com.        0 A       <ipv4_address>
+example.com.        0 AAAA    <ipv6_address>
+example.com.        0 MX    0 mail.example.com
+mail.example.com.   0 CNAME   example.com.
+example.com.        0 TXT     v=spf1 ip4=<IPv4 address> ip6=<IPv6 address> ~all
+_dmarc.example.com. 0 TXT     v=DMARC1; p=none; rua=mailto:<email>
 ```
 
 Install `certbot`.