diff --git a/mail/doc b/mail/doc
index e04cdc1..08010bd 100644
--- a/mail/doc
+++ b/mail/doc
@@ -23,4 +23,7 @@ https://wiki.archlinux.org/title/Virtual_user_mail_system_with_Postfix,_Dovecot_
 
 # do some checking
 postmap -q esin.io proxy:pgsql:/etc/postfix/pgsql/virtual_alias_maps.cf
-postmap -q hi@esin.io proxy:pgsql:/etc/postfix/pgsql/virtual_mailbox_maps.cf
\ No newline at end of file
+postmap -q hi@esin.io proxy:pgsql:/etc/postfix/pgsql/virtual_mailbox_maps.cf
+
+# dkim
+https://www.cnblogs.com/MYue/p/8205439.html
\ No newline at end of file