Question : 550 5.7.1 Helo command rejected: Host not found

A user was trying to email us, they received this error.  This user has emailed in the past and has been able to email us after getting this error.  It happened on two separate occasions about two days apart.  I'm trying to determine what the error exactly is.

The receiving email server has a good DNS report, spf, reverse DNS, etc.

I see no log of either email in our spam software, the user had to send me this returned email error, it looks like we never received it to even reject it.

Code Snippet:

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The message that you sent was undeliverable to the following: [email protected] (550 5.7.1 : Helo command rejected: Host not found) Possibly truncated original message follows:

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Answer : 550 5.7.1 Helo command rejected: Host not found

Very simply - there are no MX records setup for the sending domain.  This is a problem at their end and they need to correct it.

Check this out yourself at www.mxtoolbox.com