User name format issue connecting to NAS server (with workaround)

I thought I'd report the result of a recent support case to the community, because this came as a surprise to me and (according to the support tech).it doesn't appear to be documented anywhere.

I had some external devices that needed to connect to the NAS server on my Unity 300h.  (These are multifunction printer/copier/scanners that upload scanned files to a shared folder dedicated to this function). 

It's necessary to enter login credentials on the device, but I created a dedicated AD user account for this, with very limited access.  This setup was working fine with my old VNXe3100.

For a couple of days after the changeover, no files were getting through.  The device didn't report any error more useful than "Connection error", so  I opened the support case to get some diagnostic information on the EMC side.

In the mean time, I began playing around with settings on the printer and noticed that the user name was stored in the post Windows 2000 Active Directory format username@domain.com

So I changed the username setting to the Windows 2000 format DOMAIN\USERNAME . This caused the Unity 300 to allow the printer to connect to it and transmit files.

Support tech reported that L2 engineering  "[had] seen this before but do not believe it's a bug. Though either should work with this system, they have seen in some instances where one needs to use one format or the other. It could be a number of things that cause it to require one or the other."

What's a bug and what's not is sometimes a value judgement.  However, the fact is that the printer was able to connect to the VNXe3100 using the newer format, but not the Unity 300.

So I have my workaround and my users are happy.  But it might be nice to know what some of those "things that cause it to require one or the other" are, and if readers have any ideas please post them.