not receiving external mail from exchnage 2010

Update on the issue.

If I send an email to myself internally from the organisation, I receive it to my external email no problems.

When I reply once tot his email, it does not come into exchange 2010 and I dont get any delivery failed.

However if I hit reply about 10 times to the same email, I will receive approx 3-5 delivery message failures - see below for message error.

It appears from this that the port redirections are OK but something not quite right with Exchange 2010 setup? although was working.

message details:

            The e-mail account does not exist at the organization this message was sent to.  Check the e-mail address, or contact the recipient directly to find out the correct address.
            < nskntmtas04p.mx.bigpond.com #5.1.1 smtp; 550 5.1.1 <user@domain.xx>: Recipient address rejected: undeliverable address: User unknown>


note I have masked the user@domain for privacy - it did show the correct email address which works internal to the organisation.

use https://testexchangeconnectivity.com/Default.aspx to test internet email

@dustin99352 is correct on the receive connector
by default exchange 2010 does not enable a receive connector with anonymous access

Thanks Dustin this seems to have partially fixed the problem. If I send 4 test emails 2 of the come to the inbox and 2 are returned as undeliverable with same message as earlier.
Any ideas? Thanks

i would use https://testexchangeconnectivity.com/Default.aspx to test your environment

are all your tests sent from the same address?

The website posted above is very helpful.  Not to sound obvious, but are you sure you didn't mis-type the email address on some of your tests?  Generally if one message is getting through, all of them are.  

Thanks

i was thinking the same thing dustin, that they should all be delivered
the only other thing i can think of that may be a cause is reputation filtering

I am definately typing the correct address. I am replying 4 times to the same email and 50% of emails are not delivering.

I'll run that test and report back
Thanks

Connectivity test run and was successfully completed

do you have anything in front of exchange for antispam?
did you install the antispam agents?

there is a feature called sender reputation that may be causing your issue

I haven't installed any antispam agents at the server end but I do use dyndns for antispam but it would be safe to say as the message is comming from the server dyndns wouldn't be the problem?

i would investigate issues between the dyndns and your exchange server
check the logs there for directory lookup issues

This fixed the main problem. The secondary problem seems to only be existant for 1 mailbox as far as I can tell. all else seems ok so I will monitor and recreate the mailbox.
thanks for your help.