title: 5.5.1 501 Error, Can't Send Mail
body: <p>In the last 4 days, my mail server cant send mail to some domains. I receive these errors:</p>
  
  <p>(abc (at) aygaz.com.tr), Result: failed, Status: 5.5.1 501 Syntax error in command parameter(s) Domain name expected</p>
  
  <p>(abc (at) total.com.tr), Result: failed, Status: 5.5.1 501 Syntax: HELO hostname</p>
  
  <p>(abc (at) telemaks.com.tr), Result: failed, Status: 5.5.1 501 Syntactically invalid HELO argument(s)</p>
  
  <p>(abc (at) ziraatbank.com.tr), Result: failed, Status: 5.5.1 501 Syntax error in parameters or arguments</p>
  
  <p>(abc (at) normsan.com), Result: failed, Status: 5.5.1 501 5.5.1 HELO/EHLO requires domain address</p>
  
  <p>I looked the log files for last 2 years, but find this problem only last 4 days. I searched the error code, but cant solve the problem. Error code descriptions tell same things. "Your email adress, domain invalid, character problem etc."</p>
  
  <p>There is Kerio Mail Server 6.7 on Windows Server 2003.</p>
  
  <p>I tried to send from different IP adresses, domains, web mail/outlook and disabled antivirus program on the server but problem can't solved. Mail server ip adresses aren't blocked by blacklists. </p>
  
  <p>What is the reason of this problem, and how can I solve it?</p>
  
  <p>Thanks.</p>