ConsignCloud doesn't allow multiple accounts to share the same email address in order to have a unique identifier for any given account. For example, consignors might want to log into your portal, but when multiple accounts share an email address, we don't know which one to choose.
A handy workaround is what's known as "email tags". Basically, you add a plus sign before the @, followed by a tag, and emails sent to your modified address will still get delivered to the canonical email address. Let me give you an example:
Suppose you have a consignor whose email address is firstname.lastname@example.org. Now, Deb has a daughter named Kasie who also wants to consign with your store, but wants her records to be separate from her mother's — but she doesn't have her own email address.
This trick works with most email providers, including Gmail, but certain domains don't support it, for example Yahoo uses disposable email addresses instead. If you're not sure about a particular email provider, send a test email first to see if it gets delivered.