Add new Install_mail.md file

Install_mail.md

+** Install: **
+>sudo apt install mailutils postfix
+
+>sudo DEBIAN_PRIORITY=low apt install postfix
+
+** In configuration: **
+* General type of mail configuration?: Internet Site
+* System mail name: example.com (not mail.example.com)
+* Root and postmaster mail recipient: The username of your primary Linux account
+* Other destinations to accept mail for: $myhostname, example.com, mail.example.com, localhost.example.com, localhost
+* Force synchronous updates on mail queue?: No
+* Local networks: 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
+* Mailbox size limit: 0
+* Local address extension character: +
+* Internet protocols to use: all