Yesterday I had a client report that certain emails were being bounced
The SMTP error message was this
501 Syntactically invalid HELO argument(s)
Not one I’d seen before!
As with most support queries one of the first questions to ask is “what has changed?” and this particular client has a Sonicwall Email Security Appliance that had failed and been swapped out that same morning
Looking through the rest of the NDR it was clear the issue was with the appliance.
Generating server: spam.invalid_sonicwall_gateway_domain.com
This was also a huge pointer as to what the issue was.
The host name needs to be a fully qualified domain name but in this case it had just been set to “spam”
Once it was corrected (e.g spam.customer.com) and the device was rebooted all was well.
I’m not entirely sure how the host name ended up like that as I’m sure that form won’t allow you to enter a host name that isn’t a fully qualified domain name.
The device settings were restored from a backup so it’s possible it was part of that but I’m just making a guess on that part!
Either way it was a simple enough fix and email went back to normal after that.