Weird DNS issue

From: CHYRON (DSMITHHFX)11 Aug 2017 11:02
To: ALL1 of 6
Client needs new domain, switch primary domain on web hosting to new domain, forward old domain (on different registrar) to new domain. Client loses access to emails from old domain. Restore settings on old domain web admin to 3d pty hosting (IP & name servers). Client still can't get emails, and old domain returns unknown host in browser.
From: CHYRON (DSMITHHFX)11 Aug 2017 20:20
To: ALL2 of 6
Solved: by hosting the old domain we were able to edit the zone settings, which weren't exposed in the registrar's web admin.
From: Serg (NUKKLEAR)18 Aug 2017 18:02
Well that's just rubbish... If you (not YOU, you understand..) give a client control of this stuff, why is it only partial?!
From: CHYRON (DSMITHHFX)18 Aug 2017 20:30
To: Serg (NUKKLEAR) 4 of 6
Not really the registrar's fault, for whatever reason the guy who configured the email used zone settings on the web host (even though MX pointed at google mail servers), which I unknowingly nuked when I switched over to the new primary domain. When I added the old domain back as an additional non-primary, the zone settings re-appeared and he was able to restore the MX records. But he could have configured them from the registrar's web admin for the old domain.
From: gracia (GRACEINC)22 Aug 2017 16:13
To: ALL5 of 6
I cannot understand why clients need switches?
From: CHYRON (DSMITHHFX)22 Aug 2017 16:41
To: gracia (GRACEINC) 6 of 6
Ones who get domain + hosting from same provider mostly don't. Ours is a fooking mess.