That's just weird enough I had to go try it. I went into my admin and created a distribution list "Test@mydomain.net" and added some addresses to it. When I saved the list it changed the case for me, so that on the list it showed as "email@example.com." So I can't replicate your problem because I can't get the account to create.
I'm on 4.5.10, so this may be a fix from earlier, although I'm not finding anything in bugzilla.
So how did you get the name in there to start with?
Nevertheless you may be able to delete it from the command line:
Since this is at the Linux command line it should accept whatever case you give it; try your abd example in both case formats and see what sticks.
zmprov ddl firstname.lastname@example.org
You can find a full reference on command line utilities here.