What is Email relay or What is open mail relay?
An open mail relay is an SMTP server which is configured in such a way that it allows anyone on the Internet to send email through it, not just mail destined to or originating from known users. Email relay or open mail relay used to be the default configuration in many mail servers, certainly it was the way the Internet was at first set up, but now open mail relays have become unpopular because of their exploitation by spammers and frauds. Moreover, many relays have been closed or were placed on blacklists by other servers.
So what are the problems will come if we enable it?
If you enable open mail relay on your server then there will be a lot of problems which you may have to go through. You may get a lot of requests to allow to perform SMTP relay using the data center's Exchange SMTP server. So if you want to allow then you should worry about your server security, because some of these requested machines may not be in a datacenter where your server is placed. Moreover, if you are allowing a machine to relay SMTP messages then there is a possibility that your SMTP server will accept messages from another machine that are not meant for the SMTP Server's domain and your SMTP server will forward the message to the SMTP server which is in charge of accepting incoming messages for the beneficiary's domain.
Moreover being an owner of a server you should have authority and control over all system that are relaying messages through your SMTP server. And on the other hand, if you do not have any direct control over your SMTP server then it means you are controlling them is by configuring an SMTP connector or through configuring the default SMTP virtual server conditional settings for inbound and outbound messages.
You also need to keep in mind that relaying means that an SMTP client can use an SMTP server to his forward email messages to a remote domain. So if relaying itself is not essentially bad because as SMTP was created for this purpose.
So if your relaying is not controlled then a malicious user can use it to send bulk spam or UCE - unsolicited commercial e-mail. And by doing this it will tie up resources on the relay host and you will not be able to send any email messages. In simple words, the security risk will reject the service against your SMTP server.
You can also look into your server that who are using the SMTP authentication and you can also restrict those host who are using your relay and in this want, it will tighten your SMTP virtual server/connector restrictions.
Why HostingRaja does not support mail relay.
The reason why here at HostingRaja we do not support mail relay is that it will spam our servers. So to avoid spam emails for our server here at HostingRaja we do not support for mail relay.