Use SMTP/Mailgun for backup notifications

site-cookbooks/backup/templates/default/config.rb.erb

@@ -22,7 +22,13 @@ end
 Notifier::Mail.defaults do |mail|
   mail.from                 = "<%= node.name %> <<%= @mail_from %>>"
   mail.to                   = "<%= @mail_to %>"
-  mail.delivery_method      = :sendmail
+  mail.address              = "<%= @mail_address %>"
+  mail.domain               = "<%= @mail_domain %>"
+  mail.user_name            = "<%= @mail_user_name %>"
+  mail.password             = "<%= @mail_password %>"
+  mail.port                 = <%= @mail_port || 587  %>
+  mail.authentication       = "<%= @mail_authentication || 'plain' %>"
+  mail.encryption           = <%= @mail_encryption || ':starttls' %>
 end
 
 <%- if node["backup"]["mongodb"] -%>