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Some mail sent from sites on my server bounce back with the following mail.log message

Nov 26 17:27:53 blogu postfix/smtp[16858]: C4DD22908EC0: to=,[rejecting-ip]:25, delay=2.5, delays=0.1/0/2.3/0.04, dsn=5.0.0, status=bounced (host[rejecting-ip] said: 550 Access denied - Invalid HELO name (See RFC2821 (in reply to MAIL FROM command))

On the receiving end, my emails are logged like this:

2011-11-22 15:09:35 (Ubuntu-1004-lucid-64-minimal) [my-server-ip] rejected MAIL : Access denied - Invalid HELO name (See RFC2821

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up vote 8 down vote accepted

Ubuntu-1004-lucid-64-minimal is not a valid global name, tell postfix to use your "real" domain name, using the myhostname setting:

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Which is described in the referenced RFC: RFC2821 – mailq Nov 27 '11 at 20:55

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