Improve email address validation (#14565)

* Increase DNS timeout from 1 second to 5 seconds for MX check

1 seconds is rather short when using a recursive DNS resolver which
hasn't got a cached result already available. Use 5 seconds instead,
which is the timeout value we use for outgoing HTTP queries.

* Add more precise error messages for invalid e-mail addresses

app/controllers/admin/email_domain_blocks_controller.rb

@@ -27,7 +27,7 @@ module Admin
           ips       = []
 
           Resolv::DNS.open do |dns|
-            dns.timeouts = 1
+            dns.timeouts = 5
 
             hostnames = dns.getresources(@email_domain_block.domain, Resolv::DNS::Resource::IN::MX).to_a.map { |e| e.exchange.to_s }