My organization uses Google Workspace, say for "morg.net". Before we started doing this, years ago, a departed employee had set up user@morg.net as a Google account, and used that email address to login to Instagram to create, say, @morg-insta
I have the password for the Google Workspace user@morg.net account, and I can receive test email messages I send there. We don't have the password for the Personal user@morg.net account.
I ask Instagram to send a password reset link to user@morg.net, and it says it's done so. And when I request a reset for, say userXYZ@morg.net, it complains, so it seems Instagram recognizes user@morg.net.
However, the password link email can't be found in the Google Workspace inbox for user@morg.net (not even in spam), after repeated attempts.
This almost suggests that Instagram can send email to the Personal user@morg.net account, while other mail to that address goes to the GW inbox. But from what I know of MX records, that's impossible, since they are set at the domain (or subdomain) level, not by individual email address.
What else can I do to troubleshoot and get access to @morg-insta on Instagram?