5. Broken mail servers
The way in which mail servers send mail is defined in a set of open-standards documents. Virus writers and junk mail senders often use their own mail server software in order to deliver their viruses, worms and spam.
A lot of this custom software is broken, in that it doesn't adhere to the standards governing mail transfer. We take advantage of this fact by rejecting messages that come from servers that don't play by the rules. This reduces the amount of spam you receive.
Unfortunately, there is a tiny minority of legitimate mail servers which are broken because they haven't been properly configured by their IT staff. We can't tell the difference between badly set up "good" servers and the "bad" servers and so may end up rejecting a small volume of non-junk messages.
550-Invalid HELO (foo) used by 10.74.209.158. 550-See RFC1123 s5.2.5 and RFC2821 s126.96.36.199 550-for the correct argument to use with the EHLO/HELO command. 550 Further details at http://www.oucs.ox.ac.uk/email/external/sending.xml
If one of your correspondents is receiving this error message when trying to mail you, they should contact the administrator of their mail server and ask them to fix it. They will then be able to send you mail.