Unable to log in to accounts after DNS transfer.

I'm looking for some advice regarding email accounts that were hosted with Zoho, that now appear to have disappeared.

I work for a charitable organisation that used Zoho Mail, but has recently transferred their domain to a new registrar. We've updated our MX records to point back to Zoho Mail. However in the meantime, none of our staff can access their accounts.

They're all getting a "This account cannot be found. Please use a different account or sign up for a new account" error message.

Is this expected behaviour when a domain is transferred or should our staff still be able to log into their accounts, albeit be unable to send/receive emails until the MX record has been updated and verified?

My concern is a member of staff, who had admin user access to the account and is leaving the organisation on bad terms, has deleted the accounts under the guise of the domain being transferred.

I've deliberately not mentioned any accounts or names above, as I'm only looking for a generic answer about what happens during a domain transfer process.

I hope someone could give me some guidance with the following questions: -

Is this normal behaviour and will access be restored when the MX record is updated and verified?
Would transferring the domain to another registrar result in staff being unable to access their accounts?
Would transferring the domain to another registrar result in the accounts and mailboxes being automatically deleted or would a super user have to delete them?

I could understand the accounts not being able to send/receive emails until the domain was transferred and verified again, but I'm struggling to believe that the transfer process would remove the mailboxes and accounts.