What is mail relaying?

Mail relaying occurs when mail is forwarded from someone else through your mail server. Forwarding mail through a mail server is ok as long as either the originator of the message or the receiver is a local user. If neither the originator nor the receiver of the message is a local user, it is called third-party mail relaying. Under the hood, it happens like this:

HELO your.smtp.server
   wodSmtpServer replies with
250 wod.smtp.server
   The remote host sends the "From" part of the envelope:
MAIL FROM:<joe@unknown.place.net>
   Your host responds with:
250 ok
   The remote host now sends one or more RCPT TO commands, which specify the recipients of the message:
RCPT TO:<bob@elsewhere.com>
   hold on! elsewhere.com is not domain you want to accept messages for. So, what should you do? You can accept the message with
250 ok
   or you can reject it with
553 sorry, we do not relay

In the first case, your server is acting as a relay: it's accepting and agreeing to try to deliver a message that's not destined for a domain that your server hosts. In the second case, it's refusing to act as a relay.



What is open relay?

An SMTP server is an "open relay" if it agrees to relay mail no matter who's sending it and who is receiving it. Open relay will accept and try to deliver it to destination anyway.



Why shouldn't I be open relay?

Because of the spammers, mass-mailers and unsolicited commercial e-mail. Rather than using their own bandwidth, they seek open relays that will accept single message, and then try to distribute it to, for example, 1 million recipients - using your resources. Not only this causes a problem for your bandwidth, but it is possible that other hosts on the internet put you on blacklist for being open relay (look at http://www.ordb.org for example) and refuse to accept your messages even if when valid.



Is wodSmtpServer (open) relay?

Not at all. wodSmtpServer, by default, does NOT relay email at all. This is up to your implementation. You need to explicitly put received message to be relayed. If you don't do that, wodSmtpServer will just save the message locally.


So how do I relay messages?

Basically, it is enough to use this command in, for example, MailReceived event:

wodSmtpServer1.Relays.Add User.Message

and wodSmtpServer will try to relay received message for you. You should also set wodSmtpServer1.DNSHostname property with valid DNS server.
This is all you need if you want relaying. Optionally, you can use Relay's Hostname, MailTo, MailReceived, IdleWait.... properties to 'tweak' relaying process, if needed.



How does it work, under the hood?

When you add message to be relayed, wodSmtpServer will do few steps until message is successfully deliever, or until it gives you an error on why relaying didn't succeed. These are the steps:

1. After you used 'Add' method to add the message, wodSmtpServer will wait for IdleWait seconds before it does anything

2. It will try to retrieve domain information from address provided in MailTo property

3. It will request DNS query to retrieve MX (Mail Exchanger) records for that domain

4. When MX is found, it will resolve hostname retrieved as MX record to IP address, and connect to it

5. When connected, it will use HELO, MAIL FROM, RCPT TO, DATA, QUIT commands to post the message

If no error occurs, relaying is finished, RelayDone event will be fired and you have nothing to do.

If error occurs, then:

- if connection to server fails, it will use another MX if available - depends on DNS entries

- if there is not MX record for the domain, it will try to retrieve A record, or just try to connect blindly to domain address, assuming it's a hostname

If you want, you can change Hostname property so that wodSmtpServer connects to that defined hostname (instead of MX record information), thus using Hostname as 'smarthost' SMTP server.

No additional steps are required to relay email.