VM's email system has been through may changes over the years. The present arrangement, of having to provide a third party email address, is a mashup of fixes to try to deal with past security weaknesses. Like any bolt-on solution it can be cumbersome and unintuitive to use.
VM's email service has been beset with endless faults, and problems with reliability, as evidenced by the never-ending topics of complaint on this forum.
From 2022 VM stopped issuing email addresses to new customers and existing customers cannot create new email addresses. VM is also running an active programme of closing down mailboxes, not linked to an active VM broadband subscription, wherever it can. Regulars on here have frequently speculated that the future of VM email may not be long-lived (although VM has made no statement of intent regarding VM email).
If you have now reset your password, and have set up a third party email address, it would be a good investment of time to start moving all of your online services and contacts over to the new third party email address. This will free you, in the long term, from having to deal with the many failings of VM email.