Error sending outbound emails from single email address

I have one email address that cannot send email to external email addresses.  I get no errors when sending them but when I run the troubleshooting assistant on the Exchange server I get the following error.  Mail submission failed: Error message: Syntax error, command unrecognized. The server response was: 5.3.3 Unrecognized command.   I cannot find anything different with this email account than with any of our other accounts that work just fine.
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Check to see if you are not running out of disk space on server or user PC
PDISAuthor Commented:
I have plenty of space both on the Exchange Server and on the PC I am testing from.
Check the firewall for open ports that your Exchange uses to send email

No other ideas ... :(
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PDISAuthor Commented:
I don't think it would be the firewall or it would not be working for anybody, not just this account.
Andy MIT Systems ManagerCommented:
Does this happen when using Outlook/Webmail/phones? I would start by trying in webmail as this rules out issues with devices and applications.

If the problem occurs on webmail it is likely an account issue - possible corrupt. I would try:

1. Backing up the mailbox to pst
2. Delete the mailbox
3. re-create the mailbox
4. try outgoing email
5. import the pst (if outgoing email works)
6. try outgoing email again.

If the account works fine in webmail then it's likely an issue with Outlook/mail application/device. I would try the same account from a couple of systems that have working email accounts on them to see if the problem persists.

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MB ShaikhCommented:

Check on HUB Transport server is there any rule set for the specific user, in my case we found some rules was created earlier because there was spam incident from that account.

it is not so technical but happen some times.

MB Shaikh.
PDISAuthor Commented:
The problem does occur in webmail.  I tried deleting the account and re-adding it and I had the same problem.  I have found that if I send an email from an outside account to that email and I reply to it the reply is received.  But if I create a new email and send it, it is not received.  I checked the Transport Rules and I do not see anything that should be causing this.
Andy MIT Systems ManagerCommented:
Well, that rules out devices and indicates a server issue so the next step is diagnosing on the server itself.

Does the user's mailbox have quota limits set on it? If so are they nearly full? (Can cause issues in sending sometimes). I would also have a look at your SMTP logs as these should indicate at what stage the command is unrecognized and email refused.
PDISAuthor Commented:
I followed your advice a second time and deleted the account, rebooted the server, recreated the account and it worked without issue after that.  Thank you so much for your assistance!
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