There are a number of reasons that can cause the "553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)" error. The first item to check is that your email client is configured properly.

Here is the link to our configuration page:

http://kb.blackcoralgroup.com/kblore/article.php?id=003

Our mail servers are configured to reject mail that is not hosted on our network, you can't send mail out from our servers with a "From" address that is not hosted on our network. This is to prohibit someone from trying to fake their from address. If you have the wrong from address the server will not allow you to send the message.

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Another cause may be a misconfigured IP address at your ISP. This is especially common in Costa Rica for users of ICE DSL service.

See this page for more info: http://kb.blackcoralgroup.com/kblore/article.php?id=005
and also see below.

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Next, the DNS for your IP address may be misconfigured.  This also is a serious problem in Costa Rica for ICE clients.

FROM ONLY THE AFFECTED COMPUTER AT THE AFFECTED LOCATION (in other words this cannot be done remotely!)

1. Go to http://whatismyip.com

2. Record the IP address that is given to you.

3. Now go to: http://www.dnsstuff.com

4. Enter the IP address from Step 2 into the box labeled: "Spam database lookup" and hit LOOKUP

5. The top line marked PTR -  If it tells you that your reverse DNS has nor been configured or is wrong, that may also cause sent email to be rejected.

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Finally, you may not have an SPF record configured in your domain zone file. Some email servers are programmed to reject email from a domain not properl;y configured. Contact the company from which you purchased your domain name