Unable to receive mail in Exchange 2013/OWA

Hi all.

I'm fairly new to Exchange Server, so do forgive me if this is an extremely stupid/simple question.

I've set up a copy of Exchange 2013 on Windows Server 2012 R2 Datacenter (a clean install, may I add) I am able to send messages externally from OWA, but am unable to receive any responses.

I have run the Connectivity Tester in the Exchange Toolbox, which reports the following error on Inbound SMTP

The server returned status code 451 - Error in processing. The server response was: 4.7.0 Temporary server error. Please try again later. PRX2
Exception details:
Message: Error in processing. The server response was: 4.7.0 Temporary server error. Please try again later. PRX2
Type: System.Net.Mail.SmtpException
Stack trace:
 at System.Net.Mail.DataStopCommand.CheckResponse(SmtpStatusCode statusCode, String serverResponse)
 at System.Net.Mail.DataStopCommand.Send(SmtpConnection conn)
 at System.Net.Mail.SmtpClient.Send(MailMessage message)
 at Microsoft.Exchange.Tools.ExRca.Tests.SmtpMessageTest.PerformTestReally()


Elapsed Time: 2164 ms. 

The other issue I'm having with Exchange is that outgoing email is VERY slow to leave the drafts folder (can take upwards of 10 minutes sometimes for a simple "test" email).

What am I doing wrong? Do I need to open up any ports in the firewall? I've tried opening the standard mail ports before now, but nothing changed.

Thanks in advance;

Toby