Question : INVALID DOMAIN NAME?

Does anyone know why we are getting the following error when running SMTP diagnostics on mxtoolbox.com for "mail.arbormanagement.com"  501 5.0.0 Invalid domain name [62 ms]

Our DNS & MX records for arbormanagement.com look fine to me on dnsstuff.com

we get the following email errors rejected from certain domains;
:
Connected to 170.201.41.60 but sender was rejected.
Remote host said: 553 5.1.8 m>... Domain of sender address JStang@ArborManagement.com does not exist

Answer : INVALID DOMAIN NAME?

> Received:  from mail.arbormanagement.com (mail.arbormanagement.com [67.62.146.10])
DNS looks correct

score=3.147, required 5,
BAYES_50 0.00,
BOTNET_SERVERWORDS -0.20,
BOTNET_SOHO -0.01,
EXTRA_MPART_TYPE 1.00,
HTML_IMAGE_RATIO_06 0.00,
HTML_MESSAGE 0.00,
IP_NOT_FRIENDLY 0.33,
LOCALPART_IN_SUBJECT 2.02 --- this is only because you put exactly 'test' in the subject line
This all looks fine.

67.62.146.10 is not listed on any RBLs

It all looks good. I would however suggest setting up a SPF record and getting yourself on the free DNSWL whitelist. More info in the whitelist section of my website at http://www.gbnetwork.co.uk/mailscanner/